Hmm, I've gone underwater and breathed. It's just like normal only you don't have to worry about drowning.

Walking through walls is definitely a harder one; try going through windows or fences first, and get your hands to push through before just trying to walk through. After that walls will be easier.

Last night I changed gravity (for myself only) and walked on the ceiling. That was pretty neat.

Other stuff...fireballs/spells, flying, going around through space and at the bottom of the sea, teleporting, telekinesis, transforming my clothes/appearance, using DCs like little puppets. All pretty standard I think.