I think that some nature inspired Peel-and-Stick would be great (forest, beach, mountains, favorite city). Since your ceiling appear to be not very tall, and windows are on a small side, I would recommend choosing lighter color scheme. Try to find a pattern/theme that visually extends the ceiling height.

peel and stick murals come in all types. (I think you can even submit your own photo)

Thanks! Where are you sourcing these? I love the top two suggestions a lot!

... my favourites are the inspirational words and the palm trees agains the blue sky.

look up Motivational wall mural for gyms.

heres a couple more mural ideas

https://www.wayfair.com/decor-pillows/pdp/brewster-home-fashions-seashore-wall-mural-bzh8469.html

https://www.wayfair.com/decor-pillows/pdp/oopsy-daisy-the-great-mountains-murals-that-stick-6-x-54-wall-mural-oops6537.html

Thanks Theresa! Great suggestions!

Its a gym what are all the decor items in there , hang the TV on the wall pint the walls a nice color and add more mirrors . If you must do a mural of a place where you would ride your bike.

Since it is a basement with small windows and you have some olive greens in a nearby space, I would think about leafy themes in natural colors that remind you of the outdoors, with different shades of green and perhaps cream and tan. I like the purple mat. Maybe add some other touches of purple as an accent color. That would work with the olive green side too. I would replace the metal and glass TV stand with a wider one in a light natural wood color. You might want to make the TV a little higher in this case so it will be easier to view from the treadmill. If you put the TV on something that's more like a wide chest or cabinet, you could also use it to store some of your Fitness accessories in.