One of my favorite lunch places downtown is Cafe Claude http://www.cafeclaude.com/index.html

Also, in the Triton Hotel, Cafe de la Presse
http://www.aqua-sf.com/cdlp/

Both in your price range, for lunch at least, and walkable from Moscone. Chinatown is also a good place to go for cheap food. In my experience, the most mediocre Chinatown restaurant in SF beats all but the very best Chinese food anywhere else I've ever been in the US.

Dinner is tougher. In the lower Haight, you can try Memphis Minnies
http://www.memphisminnies.com/

Don't forget the Mission Taquerias - cheap, filling and delish. Pancho Villa is a fave, but you can't go wrong with most of them.
http://panchovillasf.com/

Nightclubs tend to be 21+ If you like live music (tending toward indie-rock), these places have all ages shows sometimes:
http://www.bottomofthehill.com/
http://www.cafedunord.com/
http://www.slims-sf.com/

Have fun!