Without getting a cab, the only thing to do is really walk down to Goleta beach (as has been said). When you land, you are forced to exit the secure area. From there, walk to the street, and make a right turn (towards the ocean) and continue walking until you get to the beach.

Construction of the new terminal should also be underway, so it's kinda neat to see how much bigger it will be. (square footage wise, I think it should be 3 times larger).