Can’t recommend Elkhorn Slough highly enough. There are two reasons it’s not on my list of favorite Bay Area hiking spots: one, it’s about an hour south of the Bay Area, in Moss Landing and, two, the best way to see the Slough is really in a kayak, though the short hikes are still worth taking.
Kayaks can be rented for about $50 from the shops at the Moss Landing harbor right off of Highway 1 about twenty minutes south of Santa Cruz. If you decide to hike in the park instead, you may still want to make a stop at the harbor where you’ll get the best views of the sea lions and otters. The park trails will not generally get you close enough to see them.