Now that you have real suggestions, I'll tell you that what he might like a whole lot is a photobook full of pictures that you guys have taken. If you've taken any of just the pretty park.
I've gotten a small one of those together into a book I made on shutterfly, and it was so fun! Of course I still personalized it with some text, but it's just pictures of the parks (not son or hubby) as a whole, and I love that I made it.
So maybe you guys can start working on that!