Dos:
Jupiter: If you're staying in Berkeley, go to Jupiter for beer and pizza. They have a dope outside area and a great selection of beers you'll never see again in whatever midwestern shithole you're living in. Other spots to check out in Berkeley include Quickly's for Taiwanese fried chicken, a hot dog at Top Dog (watch out for the spicy mustard), or a nicer option of delicious Louisiana fare at Angeline's.
The Mission: Go to Farrolito for one amazing fucking Mission-style burrito. Go to Mission Chinese for Kung Pao pastrami. Go to Old Bus Tavern for their beer selection and off-menu chili. Go to Blondie's if you're into two massive martinis for $10 and a meat market scene. The bar hopping there is extremely easy.
The Marina: Yes, it is douche bro central, but the biddies are fly and it's right on the water. Try Tacolicious for a frozen margarita.
Golden Gate Park: You won't get through this in a day, but it's about the only big city park where you're going to see buffalo. If you're into outdoorsy activities, try this or Lands End Trail.
Ferry to Sausalito: Get to see the water and peep Alcatraz without the all-day affair of actually going. Once you get off the ferry, it's bars on the water galore. I recommend Barrel House.
Dont's:
Giant Stadium: Don't go to a Giants game unless you're a big Giants fan. Yes, it is a nice stadium, but the garlic fries are like $14 and that entire area is shitty and yuppy. If you walk west from there along the Embarcadero it gets better (try the Ferry Building or the bar at Coqueta for a cocktail and view of water and the Bay Bridge.)
Alamo Square: The infamous "Painted Ladies" from the Full House San Francisco shot is currently unavailable. I live a block from there and the entire park is currently being renovated (through July).
It'd help if you talked about what you're looking for and how old you are. Great food, great bars, honeys, etc?