Just over 2.5 hours away (depending on traffic), Monterey Bay makes for a great day trip. I'm in love with the ocean, so, I'm down in that area quite a bit. There are quite a few things to do in that area:
Monterey Bay Aquarium
If you've never been, it's a full day's worth of fun. Slowly meander through the entire museum's vast grounds, spend time outside the back entrance taking in the sights and smells of the ocean, and grab dinner at one of the amazing seafood spots in the area afterward. Some of my favorite spots that you might want to linger around a bit longer:
17 mile drive
I love taking friends and family here when they visit - especially those that have seen the major sights in the SF Bay Area already. It's stunning even in the gloom. This is one of the most beautiful spots to enjoy the rocky, Northern California shoreline. Second only to my fave beach, Montara Beach (closer to SF). Here are a few tips:
Quit my non-profit career in a fit of YOLO rage. Blessed enough to go from wanderlust to wanderslut. Collecting cities & countries, not men.