For gravity change I wanted to add another video as base for brainstorming.
It's a boss battle from the game Gunstar Heroes. The stage is some sort of modern ore cart ride and your cart has the ability to change between left/right wall and floor/ceiling.
Maybe this gives some more ideas which can be carefully stolen and updated/changed

http://www.youtube.com/watch?v=Qq-5Jiq4c9M