In that same vein, I'd suggest making an effort to get to the Mission as often as possible if you're on a tight budget. Chow is a great suggestion; Pakwan is a great Indian restaurant, as are most of the restaurants on 16th between Guerrero and Mission. Nearby Chow, across from Safeway and next to Burgermeister is a hole in the wall Taqueria (marked with a big yellow sign that reads "Taqueria") that serves hardcore quesadillas rolled like a burrito. Super cheap; comes with chips.

You'll be hard pressed to find a club or bar that's 18+ in the city. Julian's at the Metreon may let you shoot pool or something, but I know that at least on the weekends it turns into a nightclub. There's also a bowling alley and ice skating (sometimes hockey) rink across the way. The other pure 18+ nightclub is one you don't want to get anywhere near - massive crowds, police patrols, and literally sex on the dance floor.

The Mission is easy to get to from your area - just walk up to Market, get on Muni and get off at Church street (for Chow or the Taqueria) or BART and get off at 15th and Mission - both BART and Muni use the same stations when in downtown, then they split off towards the Mission/Castro.