Lots to do
There are lots of things to do in SF - it's a really fun place to visit and is quite safe and pleasant once you get used to the crowds of homeless people. Good activities include a boat trip to Alcatraz, exploring Pier 39 and seeing the sealions, cycling over the Golden Gate bridge and around town, and taking a trip on a street car.
Insider Tip:If you don't want to pay $6 for a single ride on the tourist street car just take route F Muni which is a similar style of street car and takes you right to the piers from Market Street. Also google for the best deals on bicycle rentals - don't fall for the $5 off coupons which you get with the free maps.