Outdoor Cat Tree
I built an outdoor cat enclosure after one of my cats was diagnosed with gallstones. They were indoor-outdoor cats for a few years, but I had long wanted to build them something like this that keeps them closer to home and allows them to enjoy the fresh air and sun. I think next Spring I may plant around it to give them a little extra interest.
Here's something to add to the list of awesome things you can build for your cat: a catio. It's a portmanteau of cat + patio, and it's an enclosed outdoor space that lets your cat enjoy the great outdoors, without the attendant risks. If you've got the DIY itch, here a few inspiring projects to get you started. Your neighbors may look askance at your new outdoor room, but your cats will be happy, and that's what matters.