California is huge. If you could narrow down which part of California, or specific interests, it would result in better tips perhaps. Up and down we have all sorts of geography: ocean, mountains, forest, desert, urban, agriculture, etc. We also have lots of different types of people, so you could meet people from other countries you’ve never met before and try out different cuisine. There might even be a local festival going on or if you’re into music, a concert. We have tons of museums and amusement parks and the like. Naturally the biggest cities would have more to do in an urban type setting. The farm areas would have wine tasting or better selection of local produce at farmers’ markets. I recommend seeing a bit of each landscape and so you could do San Francisco to Santa Cruz/Monterey to see city, beach, redwoods (like living amongst the ewoks), ocean, and farms. Or a kind of loop of Santa Barbara, LA, San Diego, and deserts to see plastic people, crazy rich people homes, beaches, cactus, amusement parks, etc. Camp if you can. The national parks in the summer will be very crowded but if you’ve never been and you’re into it, it should still be worth it. Everywhere in California you can explore areas from your favorite films if any of them were filmed here. All areas with mountains might have snow and rednecks.

Northern California (past San Francisco up to Oregon)
never been there, I think there are trolls in the redwoods j/k
our state capital is there, Sacramento and forgive me my Sacramento friends but it’s not exactly excitement city
it’s the least populated portion of California so if you’re into nature, that’s the place to be

Central California (San Francisco to just above Santa Barbara County)
you could urban explore the Neverland Ranch
spend a weekend in Santa Cruz, where surfing was introduced to the mainland by Hawaiian princes
from SC head down to Monterey
San Jose area is silicon valley so unless you’re a geek and want to take pictures at company headquarters, not terribly exciting except for the Winchester Mystery House (widow of gun fortune a bit looney who never stopped building her house for fear of dead spirits from the guns would kill her)
if you are into record shopping, there’s several good stores in San Francisco/Berkeley and San Jose areas
San Francisco, too much to write, all internet top 10 type lists should cover SF well unless you want the obscure
or head way inland for Yosemite and Manzanar (Japanese internment camp)
or some little quaint (or kooky, it’s subjective) towns on the way to Southern California (Solvang, etc.)
some films in this area: Vertigo, Basic Instinct, Mrs. Doubtfire, Harold and Maude, Bullitt, Lost Boys, the intro to Full House tv show, etc.

Southern California (Santa Barbara County down to the border of Mexico)
Santa Barbara up to Monterey are various beautiful coastal towns that can be sleepy but pretty much always costly so you might get sick of the same thing after awhile
Hearst Castle (newspaper magnate home turned tourist attraction)
LA cliches: tour star homes, Hollywood Hills, walk of fame, Chinese and other old movie theatres, spots where celebrities died, Sunset Strip, pretend to be famous to get chased by paparazzi, landmark concert venues or where award shows are filmed
San Diego area: the zoos, museums, and lots of beach goers
there’s an Indian reservation in this area that a former co-worker highly recommended for camping but I’ve never been
also, take some time out to cross over to Mexico for the day (be sure you have your passport) just to say you’ve been there, get some cheap food/beer, dance the night away, and beware of your surroundings to ensure a happy and safe trip
films in this area: Some Like It Hot (Hotel del Coronado), Charlie’s Angels, Traffic, Rebel w/o a Cause, Beverly Hills 90210 tv show (the high school was not in BH), etc.

too many things to do… hit a couple very famous spots and try to do some things you really like and then a couple obscure things for good measure

Universal Studios, Disney Land

San Diego
1. Go kayaking through the La Jolla Coves. This is an awesome trip!
2. Visit one of San Diego’s famous attractions: Sea World, the Zoo, Wild Animal Park, or Legoland
3. Stay at or enjoy brunch at the historic Hotel Del Coronado on Coronado Island. Many famous people have stayed there and it is said to be haunted.
4. Lots of water and beach activity’s including surfing, boogie boarding, beach volleyball, jet skiing, bay cruises, fishing
5. Wine tasting.
6. Explore the museums and gardens at Balboa Park
7. Hike to a waterfall and swim beneath it.
8. Enjoy spectacular San Diego sunsets.
9. Ride a gondola.
10. Enjoy unique shopping.