Let's finish up our list with something that many parents might overlook: teaching your kids about the wonders of nature, right in your own backyard. It can be the least expensive thing of all, but it can have a huge impact. My daughter loves watching birds and squirrels, and since our property has dozens of trees, there are plenty of the little critters around. For better observation opportunities, we've acquired squirrel feeders, bird feeders and a bird bath. She lets me know when they need to be filled. Once she gets older, we might get more into bird-watching by using binoculars and a guidebook. You can even grow certain plants to attract more birds.
If you want to put in a little more work, you can attract butterflies by planting a butterfly bush. You could also buy a butterfly kit, which is a habitat that houses a caterpillar (mailed separately). Your kids get to watch it form a chrysalis and hatch into a butterfly, then release it into the backyard.
Maybe there are some wild rabbits around, like in my backyard? If you decide to plant a garden, you could include something just for the bunnies.
Composting can introduce your kids to all sorts of interesting processes, and you could even try vermiposting (composting with a special type of worm). But children can be just as happy going on nature walks; finding leaves, sticks and rocks; and observing ants and other creatures just going about their day. Don't discount the value of these simple pleasures when thinking about summer fun outdoors!