There's more than I have time for right now but...
Spend a day ambling down Hwy 1. Nice scenery & plenty to do. Moss Beach Distillery (just north of Half Moon Bay) is a great late afternoon food detour. Sit on the patio & catch a sunset. They supply the blankets.
http://www.mossbeachdistillery.com/
Give them a call for prices. We used to head over once a week to share a bucket of clams & catch the sunset. Seems to me we got out for ~$10/each sharing if we avoided a big beer tab.
Plenty of nice small beaches from there down to Bonny Doon. Old & new tourist stops, too. The earlier the better as far as crowds are concerned. For Santa Cruz, see the boardwalk but keep going to Capitola for beaches. It might have changed in the last 5yrs but the boardwalk can be tourist & teen chaos.
If you have time & tranportation, head a little further south to Monterey. Touristy, but a great aquarium.
If you're the least bit into car/motorcycle racing then know you're near two great road courses. From S.F. head north to Sonoma to see what's happening at Sears Point. It'll be expensive and chaos if there's a race but many days are just trials and lap days. I'm pretty sure it's still free admission for spectating. Same deal for Laguna Seca when you're in Santa Cruz. (track is just north of Monterey.)
Of course, if you're near Sonoma/Napa and have any interest in wines... catch a river canoe/bicycle winery tour. Look for companies that concentrate on the mom & pop wineries where you can sample & chat with the owners. Sample, weave to the next place, repeat. Cost is the tour itself plus extras if you want to buy them. Makes for a fun day. gotta go.