My kids are fascinated by camper vans and recreational vehicles. (They keep asking to go camping, but not for the purpose of experiencing nature. It's because they want to travel in a camper.) They wanted a camper van LEGO set, but instead of buying one, we went online and found the instructions for an existing camper van set and built it from the pieces we already have.
And then, of course, the kids modified the result to suit their needs. The resulting camper van looks like a lot of fun, but driving it around may be a bit difficult seeing as it has a height clearance of 30 feet.
- BrickInstructions.com has instructions for LEGO sets, in case you lost yours, or you want to see instructions for other sets.
- Rebrickable contains user-contributed instructions. You can tell it what LEGO sets you own, and it will tell you what you can build from them.
- BrickSet has among other things a part finder. One thing I use it for is recovering the name of the set by searching for pieces that appear to be unique to that set.