I just got a new XBox 360... now what?

For Father's Day, my wife bought me a shiny new drill.  While I was very appreciative, I knew that it would go unopened for at least a month or two as I had just completed the home project it was intended for.  And since we are going to move soon, it might even go unopened longer, as my old drill still works fine.

So I returned it, and got an XBox 360 from Wal-Mart.  I got the Ultimate package.  And I bought Halo 2.  And Ridge Racer 6.  I wanted Project Gotham Racing 3, but the store didn't have it in stock.

Setup was simple, although I was disappointed that the TV in my bedroom only has a cable input and no RCA jacks.  I plugged it up to the kids' toy room TV, and played Ridge Racer for about 2 hours.  I haven't opened Halo 2 yet.

So the first question is, are there any games that a 5-year old boy and a 4-year old girl could play easily? 

Really, the whole point of this post is... now what?  What would you say is a must-have for a new XBox 360 system?  I know that the 360 can serve as a media center extender... but I don't have a Media Center PC yet. 

Here is what I think I need to buy first:

What other goodies would you recommend?