ooo, what fun!!!
we had the swingset combo thing at our little house, and while it got more neglected as the years went by, our boys DID get a lot of play out of it. but since you've got lots of space and have given us an unlimited budget, let's have some fun<G>.
trees are a must. tree climbing is one of a kid's biggest joys. we had a beautiful maple at the little house that the boys loved to climb. and of course, they're beautiful and add shade and interest. so definitely some trees, a combo of fast-growing ones and some slower hardwoods that will grow up 'with' them and be part of the family forever. make sure you plant a couple of biggies where they'll support a hammock. i LOVED my hammock at the little house. we've got tons of trees here but none that are situated correctly for a hammock.
one thing that i've always regretted is never having a treehouse for our boys. they'd have loved it, and my husband could have built a beautiful one as he's talented that way. we just never got around to it. so i'm putting a treehouse in my version of your backyard, as big and creative as you can go.
can you manage a water feature? having an above-ground pool has probably benefited ME more than anyone else, but our boys did use it too, and still do when they come home. if you don't want to go quite that far, consider some sort of circulating water pond thingummy, ideally one that they can at least get into and splash around on hot days. when they're grown you can move it into a beautiful koi pond sort of thing.
my kids pushed for a trampoline, but my nerves wouldn't take it (i'm a pretty easygoing mom, but the injuries from trampolines not to mention the skyrocketing insurance premiums ruled them out for me.)
another thing i wish we'd have managed when they were still home is a basketball court. we did put a net up in the driveway, but they both loved basketball and would have loved a real paved court. we talked about it but the budget never permitted for it when they were here. but i'll give YOU one. :)
and a climbing wall! wouldn't that be fun?
and don't forget a garden. simple flower beds, and some nice raised veggie beds. all kids should have the experience of planting and tending growing things. and plan for a few benches or garden seats under the trees and near the flower beds, so you can sit out and read while your kids are whooping it up, and gardening.
i'll throw in a chicken coop, since your back yard is sort of my dream one. i always wanted to have my own fresh eggs, and chickens are relatively low work. give your kids a mini-farm experience, not to mention getting to eat what you raise between the garden, the eggs, and the chickens themselves (if you can- i think i could but my husband balks at eating animals we've tended.)
i can't wait to hear what you decide! be sure to post when you've got it planned out.
:) khairete
S.