I live in the heart of Northern California, so I can give a few tips.
1. Visit San Francisco for a day. It's actually a pretty small little city, and you can walk the entire downtown area if you are in decent shape. Visit Pier 39, but not on a weekend. The place is crowded. Get a tour of Alcatraz. Walk the Golden Gate. Take a ferry somewhere. Go to "The Stinking Rose" for dinner. This is an excellent restaurant that specializes in garlic dishes. It's easier than visiting Gilroy.
2. You said you are in the low part of Northern California. Do you have a town name? If you are near Hearst Castle, that's an excellent place to visit.
3. Carmel and Monterey are a bit further north, but they are excellent places to visit as well. There are some great Coastal redwoods in Big Basin as well. Just driving up and down Highway 1 is a pretty great experience...if you don't get car sick.
I've spent a lot of time exploring the area, as my wife and I don't have money for long distance vacations. If you are more clear about your expectations, timeline, and location, I can throw out some more info. It's a lovely part of the world.