Try to get up to Julian, have some apple pie at Mom's. Hit Dudley's bakery (Santa Ysabel) on the way.
Seaport Village is always fun (there's a seafood place - behind the Greek food place, in the court) which has some some GREAT fish-n-chips.
Walk around the Gaslamp and do some bar hopping (even if it's non-alcoholic), wath the drunks and homeless try to the scooter rentals. Always good for a laugh.
Torrey Pines - watch the hang gliders (bring a lunch to sit and relax).
Mission Beach boardwalk at sunset, ride the roller coaster at Belmont Park.
Do a walk through in Old Town, most of the Mexican food restaurants tend to overfill your plate for the price you'll pay.
Drive up to the Presidio and scope out some of the old (and beautiful) homes along the way.
Balboa Park is always enjoyable (a nice place to eat and get a couple drinks is in the aerospace museum)
Stay out of Sea World - unless you really do not like your money.