We hung a birdhouse that my son made at a Home Depot kid's workshop in one of the trees and hung a window birdfeeder so he could watch the birds.
We included a porch and patio with the house with chairs and benches so he'd have a place to sit outdoors and take it all in. And we created a new little garden bed this spring on the garage side for him to dig in and experiment. He's already asked for a strawberry pot and a little cherry tomato plant of his very own :)
There isn't much in his new garden yet (it is only April, after all), just some salvia, pansies and rose plants along with some garden sculptures (I have a love of mushrooms lately)...but he's so excited about planting annuals in it soon! (and YES, we let him do the planting - and the watering, too!) It's a awesome way to teach him about responsibility and to encourage his love of gardening.