I live very close to William O'Brian and hike there often but haven't camped there for many years. I think you'll enjoy the varied hiking trails the park has to offer. It's particularly nice in the autumn when the trees start to change.
As far as campsite selection goes, I have a couple of tools I use to help narrow down sites at unfamiliar locations. One is Google Earth and the other a website called campsitephotos.com. The latter is really nice and although they don't have every campground they do have many and William O'Brian is one of them:
William O Brien State Park Campground - Photos, Information, and Reviews
Stillwater has a lot to offer. Just walk main street, both north and south of the lift bridge, to get a flavor for historic Stillwater. The Freight House is a popular restaurant on the south end of town and for more casual cuisine we like the BC Mongolian BBQ up on top of the hill.
Enjoy your stay.