Okay, this ain't SUL at all, but some of us climb and many of us have kids.
My epiphany years ago is to NOT drop the 50- to 70-foot-tall beetle-killed spruce (to protect the house, yard, etc and then erect a pressure-treated 6x6s for the swing set, treehouse, cargo net, second treehouse, crow's nest, suspension bridge, third treehouse, monkey bridge, etc. But to just top the tree at 20-30 feet, limb it, and build the stuff onto the tree trunk.
The latest was when my 7-year-old daughter wanted a rock-climbing-themed birthday party. I topped the dead tree at 28 feet and ripped plywood to 2'x8', painted the plywood, attached some commercial handholds and some cut/sanded hunks of lumber, and lag-bolted them to the tree trunk. Three 8-foot lengths made a 24-foot-high climbing wall. There's a pulley at the top for belaying. Since this photo, I've added a second wall, 120 degrees around the same tree trunk.
It ain't a birthday at our house if the mothers aren't a little bit nervous!