defcon: Old outhouses are always a treat to come across while out exploring the desert or mountains. All the different ways they are constructed is interesting as heck. I don't know if I need one where I live, but maybe as a novelty storage shed.
Building The Outhouse As for roofing, I think corrugated metal would be the easiest to work with. Shingles are too much work. Keep it simple for something like this! USEFUL LINK WITH LOTS OF IDEAS: https://morningchores.com/outhouse-plans/
HOT TUB PROJECT: Too much work and it's a pretty small hot tub at that. Kind of nice for an off the grid home, but not the best thing for a normal home.
How to Install a CHEAP Boulder Retaining wall with just a skid steer and scrap field stones This guy gives a lot of good general tips that will help people design and build stone retaining walls.
Building Stone Retaining Wall // Simple DIY Dry Stack Raised Bed The last video showed a wall using large stones that required a machine to move. This one shows a wall built with stones that can be lifted by normal humans. Much more in line with what someone might do as a backyard improvement project.
Off grid gazebo with fireplace! This gazebo/shack was built out of all recycled materials. It looks a little dumpy, but it has some nice details worth taking a look at.