I've always loved the Ethel M's factory tour. When I was a kid my grade school would sometimes take field trips here, I guess it was a nice, free place to take us that was exciting for the kids, being as its all about chocolate!
As an adult I still like taking the quick tour, followed up with a free sample or two. Of course, I usually end up buying a bunch of chocolate to take with me, but hey, that's why the tour is free, right?
The cactus garden is great as well, nice place to stroll about while feasting on chocolate goodness such as the raspberry buttercremes.