Harmonica, you can sing through the thing to make some truely freaky sounds. Also it's the only instrument I know of that can easily fit into your pocket.
Gongs are fun too, but somewhat limited. Can't really do much besides build up the ringing sound and just making them crash.
Tubular bells are also a lot of fun to play.
Xylophone and Marimba's, basically the same, but slightly different. Marimbas tend to be wooden and have a sharper, crisper sound. Xylophones tend to be made of metal and ring much more when played. More or less, they're just huge piano's, keys are all even the same and everything you learn on one basically transfers to the other, just more fun to hit things with mallets than to tap keys with your fingers though.
Steel drums, or kettle drums as some people call them, very nice as well, and pretty hard to master, certain spots on the drum produce different notes and nothing is easily marked, so you pretty much have to memorize where each note is on every one that you play.
Drum kits, again, tons of fun. Your standard rock drum kit (usually consisting of a crash cymbal, ride cymbal, bass drum, hi-hats, floor tom, snare, and tom-toms) alone is a lot of fun, but if you have the money and drive to go crazy with it, well, it gets even more fun. Add in some cowbell, a double bass drum, a chinaboy cymbal (has a bit of a gong-y sound to it), a splash cymbal, and a tambourine, and you've got yourself a party. Seriously, drum kits are fun for nothing more than the customization, you can seriously make it with just a bass drum, snare, and crash cymbal, or you can go all crazy and add in pretty much anything you can imagine. Then there's the fact that you can switch out your drumsticks for say, wire brushes(can't remember if that's the actual name for them or not), and play with those, and you can get a whole new sound out of your kit.
Oh, and then there's tuba. The only instrument that I can think of that can easily be used to make fart sounds. You know a guy invented it.
Man, I could go on and on here about fun instruments to play. Most of them would be percussion instruments, but what do you expect, hitting stuff with sticks to get music is awesome.