Santa Cruz is cool, but you probably won't spend more than half a day there though. Don't miss Giant Dipper. It's a classic. The tunnel is the coolest thing ever. The beach itself is pretty nice, but you should walk south a little past the boardwalk if you just want to relax because it gets quite crowded. The area is a pretty eclectic place, much like Venice or the Santa Monica Pier down south. It's just Norcal's version of it.
