I hear ya on grandkids! We're lucky that our 2 live close to us here.
Plenty of boondocking here in CO (oh wait - don't say that on a public forum!) There is NO camping in CO! We're full! :)
I believe all the National Forests/BLM allow boondocking - at least I've not run into any areas that didn't.
Next step up is plentiful primitive NF/BLM campgrounds, and a bunch of State Park campgrounds, too. You just have to drive a little ways from the Denver Metro area to get away from the crowds, but not bad, really.