Go to beach and surf, the water should be warm depending on the month.
You have no chance at the bars, they're strict. Nightlife is centered around the 21+ crowd but if you're in Mission you can probably get some house parties going with other vacationers.
If you're staying on the board walk you can probably just pull in girls from there.
Eat Mexican food at Oscar's.
TJ is like 30 minutes from there and I wouldn't suggest driving in because the border line can be 4+ hours coming back. All you need is a passport to walk across, the line can be kind of returning unless you have global entry. Some good clubs, but can be sketchy if you don't know what you're doing or speak spanish or are going with a local.