Wilmington DE is actually a great place to spend a relaxed weekend poking around scenic and historic homes, sites, gardens, etc. Most of these are in an area known as the Brandywine Valley. Of the sites listed in that link, some of my favorites are Hagley Museum, Longwood Gardens, and the Brandywine River Museum. My (great-) nephews and nieces love the Delaware Natural History Museum. And while it's not technically in the Brandywine Valley, Old New Castle is a wonderful place to stroll from one historic building to another and listen to the docent-led talks.