Lots of interesting old buildings and the Torpedo Factory, a community of artists in the WWII torpedo factory right by the marina.
Heck -- they are almost in my neck of the woods (or neck of Virginia). When they sailed up the Potomac, My neck was on their port side.
If they want something a bit more funky, they can pop into Colonial Beach on the way back down the river. For great ice cream, send them to Chitterchats in Reedville.