Can take a day trip down to mexico. Depending on where in San Diego you are, maybe 20-30 min to the border.
Nunu's is a pretty cool dive bar. I only been there once though. drinks were cheap and strong. The casbah's kinda cool too. was there only once for like an hour.
Pacific beach has a lot of 20's olds many from college UCSD/SDSU. drinks are kind of weak though. Other than that, Gaslamp as Xericx said.