Take MUNI to Embarcadero and Green St. It's time for the Exploratorium! I've never met anyone who doesn't like this museum. It's all about science and it's totally hands-on. Cost: $29.95/adult, $19.95/4-12, under 3 free. They have 6 free days per year, but the last time I went on a free day it was so crowded I would have preferred to pay. Hours: Tues-Sun 10-5, closed Monday. The Exploratorium actually has a pretty decent cafeteria called The Seismic Joint Cafe (get it?).
After you're done at the Exploratorium (take your time, it's awesome in there) walk along the ocean and view the Bay Bridge. Check out the dancing roller skaters and street performers. At night, see the Bay Bridge light up with an art display that was supposed to be temporary. People liked it so much they never took it down! Grab some dinner in the Ferry Building. Everywhere in there is good, albeit a bit expensive.
One more thing that’s worth a mention in this region is Coit Tower. It’s a big art-deco monument shaped like a hose to commemorate all the fires that happened after the 1906 SF Earthquake. Watch for the wild parrots of Telegraph Hill as you hike! Cost: Free.