My husband and I turned a portion of our unfinished basement into an exercise space -- basically concrete walls and floors, but with a full size window and plenty of light. We got a berber carpet remnant for the floor. I painted the floor and walls in colors I love (because I need all the encouragement I can get down there), albeit the color choices were limited because I needed to use a special paint for concrete. I did the floor in a creamy yellow, and the walls in a spring green. I also have a large piece of batik fabric with a dramatic and colorful tropical scene, which is mounted on a piece of foam core, to decorate the space. Despite the drama of the "art", it's a pretty mellow looking space. (But I still need to force myself to go down there to use the elliptical...).

@Emily H @Bryan Anthony @Becky Harris Any updates on your Houzz TV Live series? If this has not happened yet, then please consider posting it. People might be interested in gyms, as part of new year's resolutions ;) For my part, I would like the gym to have a view of the garden as shown on your OP. I was considering a combo of (1) an ADU/shed or converted shipping container and (2) placing it under the deck. Any photos of similar solutions would be welcome.

Much may depend on what colors the rubber flooring comes in. Do you have pix of that you can post? It will be a major part of the whole, and there will probably be a limited offering of colors, so I think that may be your starting point.

Thought this article might help. Consider your flooring and if there is natural daylight in room, your favourite color to help you feel relaxed or energized. It’s really a personal choice as different colors affect people differently, but article has suggestions on color https://justourviewfromhere.com/how-to-create-a-super-zen-home-yoga-room/