Delaware City is worth a walk-through, if you like rundown historic buildings (like I do). Fort Delaware on Peapatch Is, take the state's boat from Delaware City.
New Castle, DE, a lovely, walkable historic town. Some (few) Colonial buildings and museum houses; mostly 19th c. No docks; you'd have to dinghy in from near the NCYC's moorings south of town.
Kalmar Nyckle is docked on the Christiana River. Probably possible to take a cab into Wilmington from there. Not much to see in Wilmington but the surrounding area is pretty full of good stuff: Hagley Museum (DuPont black powder plant), Winterthur Museum (HB DuPont house and American Decorative Arts, and the gardens, Longwood Gardens, nearby Chadds Ford PA, Chadds Ford Museum (lots of Wyeth).
Nothing until Philly, unless you can reciprocate into the Corinthian Yacht Club in Essington.