if you do a houzz search for "home gym" you'll see some sensible ideas and some crazy ones.

when my daughter was young she danced, and i did mirrors along a wall for her, it made the room 2x as large visually which helped as her furniture was all crammed in the corner to leave floor space.

i'd like your space better without the carpet, i would feel uncomfortable being sweaty in there.

My biggest dilemma is what to do with this built-in desk. Photo 2 below. It is approx 42" wide and 28" deep. I guess I could keep it and paint it. Then add a new countertop. (OR maybe remove the desk and just have a countertop). Put a dry erase board above it. Add a wall mounted light to replace the bar of lights. I like photo 1 below. Any ideas are VERY welcome - colors, materials, etc.

Keep the desk, paint it if you like. I would use that for the TV. Face the treadmill that way if you are going to use it while on it. The desk can store things like towels, small equipment and such. I know it's tempting to get rid of the carpet but in a second story room hard flooring will be very loud. Get equipment mats for under the treadmill and bike and use several yoga mats for yourself.

Kingseed what did you end up doing? I'm in a similar dilemma!