Postcards are so much fun to receive. There are kids out there who are terminally ill. They will never have the opportunity to see the world. There is an organization called Send Kids The World that organizes collection of postcards for severely ill children. For the cost of a post card and a stamp your child can show these children where they live. It's a great way to teach children to give just because somebody else needs something.
Make your own or pick up a box at the store. Head on out to the dog park. You and your child will make lots of new friends both human and canine.
Go to your local library and pick up some books about plants, animals, or birds. Let the kids start identifying them. You might want to supervise this one. I mean who doesn't need to know the difference between English Ivy and Poison Ivy. It can save a lot of misery in the future.
What are some of the inexpensive activities your kids participate in?