Slide with a beer bottle down the neck and take a sip when you get past the nut, make it look incidental that music's happening (it'd sound that way if I tried it!)
I saw a guy using his music stand as a slide once and jumping around tables as he played. He kicked ass.
There's always tapping and, if you swap your hands round when you do it people think it's harder for some reason..
There's two handed bending, you bend a bit with one hand and then put the other hand behind your fretting hand and push the string around, you can get 3and a half tone bends that way if the string doesn't break!
What'd really impress ME would be seeing someone bend two or more notes different amounts at the same time, that's a killer!