Very cool. A lot to see and most of it is free (but make sure you do the paid house tour of Clayton because that is the best part).
For free you can see the art museum, the garage the houses the families cars and carriages, the greenhouse, and the grounds. For a small fee you can take a tour of the amazingly beautiful and well preserved house. The tour lasts 1 hour, is very informative, and you get to see a lot of the rooms inside and have the opportunity to ask questions. Make sure you reserve for the house tour online because they do sell out.