Five Islands is fabulous. If you search google images, you'll find some pretty spectacular photos for five islands lobster maine http://tinyurl.com/7sh5mmh That's not a lot of exaggeration in photoshop, it really IS that pretty.
There are some small art / pottery galleries in that area, but not an awful lot else besides Five Islands and the State Park. I'd scoot up to Bath and use the rest of my day there.
That being said, beach-wise, I'm partial to Popham Beach State Park, which isn't that much farther, maybe 15 more minutes. And if you're going to Bath anywhere (you should), Popham is very easy to get to. Phippsburg has some lovely old architecture. And Popham has an old fort to explore.
Suggestion: the amount of exposed beach (at either place) varies wildly between high and low tide. Try to time your visit if you can to coincide with low tide so more of the beach will be exposed.
Edited: 22 May 2012, 8:54