
If you're flying all the way to CA, you might as well stay and visit for a bit. Here's a list of places that you might enjoy visiting!
Santa Cruz

If you're staying in Santa Cruz for the wedding, you'll have some time to check out the local sites. Santa Cruz boasts some lovely beaches (Surfing! The Pacific Ocean is cold!), a Boardwalk (Junk Food! Rides!), and some lovely Sequoia forests (Hiking! Birdwatching! Forest Fires!).
Santa Cruz is also home to the lovely and cheesy marvel that is the Mystery Spot. Everyone who visits gets a free bumper sticker- on the right is Jen's cousin Mariah proudly displaying hers:
Monterey
If you like aquariums- the Monterey Bay Aquarium is about an hour south of Santa Cruz and well worth the visit. They are currently running a special exhibit about seahorses and leafy seadragons that is phenemonal.
Near our house (San Jose/Pennisula)
The Computer History Museum has a ton of cool old computers, including many "artifacts" you may have used yourself, like a Speak & Spell or a Coleco Adam. Its also located right near the Google Campus in Mountain View.

The Winchester Mystery House in San Jose is a huge sprawling house that is fun to tour. The house is the result of the Winchester rifle fortune combined with a slighly crazy old woman and lots of time.
If you're interested in aviation, the Hiller Aviation Museum is located at the San Carlos airport, and has a good number of aviation-related exhibits, as well as fully restored aircraft.
San Francisco
There are hundreds of things to do in San Francisco. Some of our favorites are:

Touring Alcatraz. You'll want to purchase tickets in advance, as they tend to sell out. The audio tour that was created for Alcatraz was very well done, and if the day is clear you get some phenemonal views of the bay. Also, the ferry leaves only a few blocks from (but not at!) Pier 39, home of all the SF souvineers, sea lions, and some of the most expensive but quite delicious clam chowder.
Alcatraz isn't the only interesting island in the bay, you can also take a ferry ride to Angel Island. Angel island has a lot of old buildings to explore, as it was once an immigration station, a Fort during World War I and World War II, and a Nike Missile Site. The island is also home to a ton of wildlife and has several hiking trails and campsites.
If you're in San Francisco you'll probably want to go to Golden Gate Park. There is a ton of stuff in the park including some neat museums, such as the DeYoung and the Exploratorium. The California Academy of Sciences has recently been redone and is especially interesting.
And, of course, you can also take a ride on a cable car, a San Francisco icon. (we recommend getting up early to do this -- the lines can be murder). If you want to learn more, you can go to the Cable car museum.
California

Yosemite National Park is gorgeous and not super crowded in May. Its also likely covered in a fair amount of snow. If you do decide to visit, you will want to make sure you have snow chains, as they might be required on various mountain roads.
Lassen Vocanic National Park is one of our favorite national parks. It is rarely crowded, which means you can spend more time checking out the wildlife and the geothermal activitity and less time stuck in traffic or yelling at children for feeding the squirrels. It is also likely to be snowy in May, so check the weather before you make plans.
If you're not up for a 3-4 hour drive out from Santa Cruz (but not from San Francisco!), you can also look at Muir Woods National Monument, which has some short trails through sequoia groves.