You can get long balloons that you blow up using a little tube. When you let them go they fly off in any direction making a farting noise. What funnn! Anybody who complains about these is a proper party pooper.
At my 21st a couple of years ago i put on every table a selection of interesting snacks. They included drumstick lollies, jelly babies, flying saucers, mojo's etc. and i also put a rubix cube and Jenga on alternate tables, it was quite funny to watch peoples reactions change as they go drunker!!
I went to a 21st and they had condoms on the tables! The funny ones that have different animals painted on and ears etc. By the end of the night there were blown up condom cows, cats, tigers etc all over the place!