HA! Grav bongs are great! WE did build a giant 3 chamber bong out of some spare tubes we had from bunny proofing our living room (bunnies are the most fun things when you're high - since they're so timid, they love stoners, because they're basically softly spoken lumps). That was pretty cool, it even had a rice chamber (if you puff a bong through some rice, it cleans out the smoke a lot) and was smooth as fuck. Unfortunately I stood on it, so there goes that...

Another fun thing is mixing 50/50 skunk and sheesha tobacco and smoking that through a hookah - it is really efficient (a sliff's worth lasts about half an hour) and lets you play smoke while you think (like the drinking game, but you know... smokin').