Rehoboth is pretty solid, at least in combination with surrounding beach towns. It's got enough that there is a lot happening, but it's easy to find a more secluded stretch of sand.

If you're looking for the party-type beach environments, head for the main Rehoboth boardwalk or adjacent Dewey Beach. The National Seashore to the south is more chill, and the Rehoboth Bay to the west gives some great boating/jetskiing opportunities.