Single-use tickets purchased from subway vending machines are valid only on the day of purchase for use in that station. Do not buy your return ticket at the same time as your outbound ticket because it will not work. This detail is clearly explained on the ticket that you receive after you have paid for it. (Also, the vending machine will ask you how many "sheets" you want. It's asking how many tickets you want.)
Subway station names are printed in both Chinese and pinyin, but the pinyin omits the tone markers, which means that you will have no idea how you're supposed to pronounce the station name, should anybody ask you to say it. (See below.)
Unlike in some cities, where the subway logo is bright and distinctive (and therefore easy to spot from a long way away), the logo for the Beijing subway is not consistent in color, which makes it hard to pick out from a busy streetscape. (One sign I saw used the high-visibility color scheme of beige-on-brown.) The logo is a monogram of the Latin letters "D" and "G", because the Mandarin word for "subway" is pronounced "dì-tiě". You might think they would use "D" and "T", but you're being too literal.
Even worse, the Olympic Green station entrance nearest the convention center is so unobtrusively marked that if you aren't standing on the correct side of the building looking directly at it, you won't see the sign at all and you will end up spending an hour walking around Beijing looking for it. (For reference, the station entrance is opposite convention center entrance C-2.)
When you fail to find the Olympic Green station entrance, you might consider going to a security guard booth (they are all over Beijing, the city being somewhat security-obsessed in a mostly-theater sort of way) holding a subway map with the Olympic Green station circled and asking, "火車?" because you don't know the Mandarin word for "subway" and have to make do by asking for the "train". Do not expect the security guard to have any clue what you're asking for. (Okay, I sort of undermined myself by pronouncing the first word in Mandarin but the second in Cantonese, because the two languages occupy similar portions of my brain and I often get them mixed up. But still, the first two cues...)
In general, if you ask for directions but don't know more than a few dozen words of Mandarin, you're going to be in a world of hurt. My plan was to hold a map, point at it, and ask, "我在哪裡?" ("Where am I?"), and then calculate what direction to head based on the answer. Do not expect people to answer the question you ask. They will instead ask you other questions like "你去哪兒?" ("Where are you going?") but since answering that question is beyond your vocabulary, all you can do is repeat your original question, and they will give up, frustrated. Telling them that you're from the United States doesn't help, because they don't speak English.
(The "Where am I?" technique worked great in Germany. The person I asked would locate me on the map, and even orient the map, then follow up with additional questions that I struggled to understand and answer.)
It has been suggested that my profound difficulty in getting directions was exacerbated by the fact that I look like somebody who should know Mandarin. If I were some European-looking person, I wouldn't have had as much of a problem because, I'm told, Chinese people naturally assume that if they see a Chinese person on the street, that person speaks Mandarin. (The person may speak a regional language as well, but you can count on them speaking at least Mandarin.) If you look Chinese but don't speak Mandarin, then they will just get frustrated at this Chinese person who refuses to speak Chinese.
This roughly matched my experience. Pretty much everybody assumed that I spoke Mandarin. The exception? The street hustlers and scam artists. They had me pegged for a foreigner.
By the way, I eventually solved my problem by looking for a bank. The manager on duty spoke some English, and combined with my rudimentary Mandarin, Cantonese, and Hokkien (thank heaven for cognates), I was able to get the information I needed. Of course, by that time, I had wandered so far astray that the nearest subway station was nowhere near the one I was looking for originally!
Oh, and do not expect the hotel concierge to give you an up-to-date map. The information on the map had not been updated to take into account recent subway expansion, which means that its directions on how to get to points of interest were unnecessarily cumbersome. (What's more, the hotel itself did not appear on the map, because it was covered by an inset of the Olympic stadiums. This makes it hard to orient yourself once you step outside.) In fact, most of the time the street I was standing on didn't appear anywhere on the map (or at least I couldn't find it), so I had no clue where I was.
The air pollution in Beijing is legendary. The week before I arrived, the United States Embassy declared that the Air Pollution Index in Beijing topped 500, earning the rating "Hazardous for all people." On the other hand, the official Chinese government pollution index was "only" 341. (Mind you, 341 is still off the chart. For grins, compare the scales used by mainland China, Hong Kong, and Malaysia.) You don't really notice the effect of the air pollution until you return to your hotel at the end of a day outdoors and wonder why your throat is sore and you feel like you spent the day in a smoke-filled bar.
Walking through the Forbidden City makes you feel like Frodo in The Lord of the Rings. You fight your way across a courtyard to reach the building at the other end. Upon reaching the building, you cross the gate and before you lies... another seemingly-identical courtyard. This repeats about twenty-five bazillion times. They should really call it the Forbidden County.
If you're trying to get to the Summer Palace, do not accidentally leave your map in your hotel room thinking that "This is such a prominent tourist location it must certainly have adequate signage, or at least be present on the 'things nearby' map at the station." The only nearby attraction on the map at the station is the Old Summer Palace. The only directions to the Summer Palace is a single arrow on the plaza level of the station. The arrow tells you to Frogger across a busy four-lane street (and over the fence). If you go to the crosswalk some distance away, you end up wandering in the wrong direction for a while and turning around when you figure "This can't be right." On the way back, you try a slight variation on the path out of the station and notice that there's a directional sign for the Summer Palace facing away from the station. (I.e., only people returning to the station can see it.) You follow that arrow and wonder if you're on the right street since it's pretty much an empty street as far as the eye can see, but you gradually find tour buses so you figure you're getting closer. You then find the Summer Palace parking lot and say, "Cool, there must certainly be a sign to the Summer Palace from the parking lot" but you'd be wrong. You then see a tour group in the distance and take your chances that they are going into the Summer Palace (rather than returning) and follow them down an unmarked side street, then another unmarked side street, before spotting the entrance to the Summer Palace.
China clearly has yet to figure out this "foreign tourists not visiting as part of a guided tour" thing. My guess is that since it was a closed country for so long, there was no need for directional signage because all foreign tourists were necessarily accompanied by a government-approved tour guide, and the government-approved tour guide knows how to get there.
The Summer Palace is very scenic. I bet it's even prettier in the summer.
Orthographic note: Out of habit, I use traditional characters even though China uses simplified characters. With one exception, I'm reasonably comfortable with reading both sets of characters, though when writing I prefer traditional. Traditional characters feel more formal and "standard" to me, whereas simplified characters feel too casual for normal use. (Like writing "u" and "b4" instead of "you" and "before".)
The one exception? The character for "car": 車. The simplified version is 车 which to me is unrecognizable because it destroys the ideographic representation of the top view of a car. (The central box is the body of the car, and the horizontal bars at the top and bottom are the axles. The simplified version is just a number "4" with some extra bars.)
Mostly-theater: X-ray machines are omnipresent, but they are largely ignored. People just walk right on past them. The TechEd conference I attended had three security checkpoints: One with an X-ray machine and a metal detector, and two additional checkpoints where security personnel checked that you had a valid badge before letting you pass. What nobody appeared to notice is that if you took the publically-accessible skybridge from the Intercontinental Beijing Beichen hotel next door, you could enter the inner sanctum of the conference without ever passing through a security checkpoint.