Go out to eat for free — really! Search restaurants in your area with free meal deals for the kiddos.
Go birdwatching (download a cheat sheet online or borrow a book from the library for some guidance).
Make a family tree.
Play school (because you know a tiny part of them is missing it!).
Go for a bike ride.
Check out one of The Home Depot's free workshops for kids. They happen the first Saturday of every month, from 9 a.m. to noon, in stores across the country.