Life is a little more fun when you know some Japanese, whether it's to communicate with nihonjin who can't use English or to catch the attention of cute Japanese coeds on campus. One word you can get quite a lot of mileage from is sugoi (soo-GO-ee), which means "amazing" or "incredible" and which can be used just about any time you need to praise someone. See a Japanese man who's adept at playing the Super Mario theme on an accordion? Just tell him sugoi! and your meaning will be instantly communicated. Then there's daijobu (die-JOE-boo), which means "okay" or "alright" and is generally one of the first words a person learns after coming to Japan. Want to ask if someone is alright with your menu selection at a restaurant, or if they're okay after bumping into that drunk salaryman just now? Just say Daijobu? (Are you okay?) and they can reply, Daijobu (Sure, I'm fine). Finally, another fun short word you can find some interesting uses for is zan-nen (ZAHN-nen), which means "what a shame!" or "how unfortunate!" If someone misses the answer to a question or otherwise fails at something, you can pull this word out and get some laughs from them. It's used a lot by Japanese game show hosts when a guest makes a wrong answer and misses out on the big prize.
Daijobu! It's okay!