I'm not sure of the exact commuter rail roundtrip charge, but I'm positive it's more than $8 to Rockport/Salem.
At any rate, keep in mind that there can be gaps in service outside of rush hour, and this is even more true on the weekend. Read your schedule carefully.
Rockport is a really cute town. The center is very walkable, and has many cute shops and places to eat, plus a couple of small beaches. Halibut Point State park has a quarry and stunning views from the rocks over the ocean. You could probably do a little wading from the rocks that are closer to the water, but it isn't a good place to actually swim. The park is a long walk from the commuter rail, but there may be local bus service.
Manchester-by-the Sea is also another nice town on the same commuter rail line. It has a very picturesque, swimming beach that would be about a 10 minute walk from the commuter rail. I think they charge a small walk-on fee. There's good ice cream at Captain Dusty's across from the park area in the center of town.
I like the Peabody Essex Museum in Salem, particularly its "Chinese House" but personally, don't care for the "witch" sites.
Hope you have fun, whatever you decide to do!