There are lots of fun things to say. It's also easy to get a laugh by using out-dated word or unusual words the Japanese don't expect gaijin to know...
ええやろ。 It's okay. (my classmate says this in a low, husky voice so it seems like he's saying あいしてる to our teacher)
なんでやねん! Osaka-ben for "What the heck!?" "what are you on about?" or "what'd you do that for?"
なんでみてんねん? Osaka-ben for "why are you looking at me!?"
さらばじゃ! An older way of saying good-bye. it's pretty casual, so only use it with friends. (don't say it to your gf's father... *cough* :whistling: )
おれはきちょめん。 "I'm detail minded. / I'm anal retentive." Actually one of the first words I learned here.
大丈夫 - だいじょうぶ。 It's okay. I'm okay.
大丈夫?- Are you okay? Is it okay? Probably the first word I learned here. Amazing the things you don't learn at school, or from books, that are so important. You can use this in so many situations, it's not funny.
まじで? I can't believe it! Really?
Lots more, but these were the ones that sprang to mind.