My son is still really enjoying creating things in games. He spent a bunch of time making characters and armor in Scribblenauts Unlimitted and showed me how all the steps of the process worked.
When you design parts, like a helmet, you can then combine them into your larger creations. You can also share your designs with other people online.
He has also been creating characters and games in Little Big Planet 2. He designed 3 different levels for a multiplayer battle game he created. We played together until my thumb gave out, lol.