Winterthur Museum, Garden & Library

It sounds like a palace for vacationing Norse gods but Winterthur was formerly the 979-acre home of the du Pont family (one of the many du Pont estates). H.F. du Pont was an art and antique collector so his legacy includes a mansion-museum filled with 85,000 objects of note. At $30 to $40 per person, this isn't the cheapest thing to do in Delaware, but you do get a nice 1- or 2-hour guided tour of the estate.