If you have some extra time in the area, Longwood Gardens is an hour away but worth the drive! We've been members for three years now and love stopping by every chance we get. Via Wikipedia: Longwood Gardens is a public garden that consists of more than 1,100 acres of gardens, woodlands, and meadows in the Brandywine Creek Valley in Kennett Square, Pennsylvania, United States. It is one of the premier horticultural display gardens in the United States and is open to visitors year-round to enjoy native and exotic plants and horticulture.
Especially if you're flying into BWI, we recommend stopping by the National Aquarium! This is Michael's favorite aquarium, and has a bunch of great exhibits, including a "Great Barrier Reef" recreation and touch tanks for not just Delaware's famous horseshoe crabs, but moon jellyfish as well! The National Aquarium has also added an outdoor wetlands exhibit, bringing lots of native wildlife back into the Baltimore Harbor. If you're lucky, you'll see some otters!
Relatively close by, and you can rent a canoe or paddleboat for cheap and spend an afternoon on the water!
Grab some ice cream at Jessop's Tavern and walk by the Atlantic in Battery Park, enjoying the historical architecture and cute shops in this area of Delaware!