I dont know if you consider this northern California, but yosemite national park always makes a fun trip. There's so many things to do and sites to see there and they are all so amazing. That is my favorite park ever, those trees
will truly blow your mind! I hope that helps :)

Sea Ranch!

If you like wine, Sonoma County.

Mendocino (and all of the surrounding towns) are my very favorite place in Northern California. It's about a 3-hour drive from SF, and there are dozens of great little inns and bed & breakfasts to stay in - or you can go camping or rent a vacation house.

We go once a year with a bunch of friends and rent a house on the beach for the weekend. Check out Mendocino Coast Reservations - they've got really nice rentals, and if you go in the off-season it's incredibly cheap...

Tahoe is so much fun. In winter there is snow and in summer you can swim in the lake. Santa Cruz is fun – boardwalk & beach. Mono Lake is amazing. You can visit the Redwoods. Take a road trip up (or down) the coast. Some friends and I do this quite often and just explore new places. We ended up at the Lost Coast Brewery one night. That was an interesting place.

Anyways.. you’ll find something to do.