My partner and I have been talking about making some cat paths in our house since long before we owned our house. We recently got 2 new dogs, so now it is a high priority project. The first step was putting up some shelves in the hallway and making a portal from the hallway to the bedroom closet (you can see it just above the tent). The surround for the portal is this 3D printed tunnel
https://www.printables.com/model/3068-cat-door
The next project is going to be putting a portal from the spare bedroom into the stairwell so they can go up and down floors without the dogs being in the way
We were thinking about doing a hammock in some other spots in the house. One of our cats loves the one on their big scratching post