My 2 cents here. I'm from Monterey County, so I'm biased. But Monterey has
so much to offer. I'd definitely do at least 3 nights here, rather than in Sonoma. I like Sonoma, but...
While here, you need to go to our world-class
aquarium. Visit Cannery Row, made famous by John Steinbeck and home of the early oceanographer Doc Ricketts. Drive the
17 mile drive in Pebble Beach, a beautiful drive. But for a truly breathtaking drive, you
can't miss driving down California Highway 1 through
Big Sur, which is only about 25 miles south of us. It's a world class drive.
I agree with the others that the ocean off northern & central California is only swimmable by children, teenagers, and seals. Still, hanging out on the beach is a great thing to do on a nice day. Too bad you're coming in July, our cold season (fog). The weather is much better in Sept/Oct.
If you want to go to the beach, your best choice is to hit the
Santa Cruz Beach Boardwalk on your way down (or, it's about 45 minutes from Monterey). The URL above shows the amusement park section, but it's also a great, long stretch of beautiful beach. But if you're already in Monterey, I'd go instead to Carmel beach, which is about a mile of beautiful white sandy beach. The waterfront at Monterey is also a good beach. If you have little ones, the best protected beach is Lovers Point in Pacific Grove. You'll frequently see parents with small toddlers there.
HTH,
Bob