How can I decorate a basement cinder block space-> exercise room?

I would soften up some of the surfaces, you can use a few rugs with good back gripping, maybe set your equipment on rugs, or place one beside each piece, you could add a few soft chairs with pillows and brighten up the space with like an ocean blue wall and paint the rest sand color. It would be so pretty and relaxing. A few peaceful ocean pics on the walls and you'll have a paradise to work out in.

You might could use fabric wall hanging to soften the space.

You could even make a scrap wood wall like this to add a natural element. https://www.hometalk.com/diy/decorate/walls/how-to-install-a-scrap-wood-wall-3254469

Or if you can find old windows, they could be fun https://www.hometalk.com/diy/windows-doors/windows/custom-stained-glass-28820149

You can use fish net on the wall to start with.

The magic of paint! Here's how I updated one: https://bachelorettepadflip.com/2017/08/04/flip-4-friday-feature-the-basement/

Hi Jules, you can get mats to out down that have some cushion like many gyms do, it's warmer on the feet. The cinderblock can be painted, it will look much cleaner once it is. you can also add posters or pictures to inspire you, You can add narrow bookshelves to store weights on, Here is a link with ways to hang things on cinderblock, hope it helps you out, https://www.thespruce.com/ways-to-attach-things-to-cinder-block-walls-5080521

You can paint the floors and walls with cement paint. If this is not possible, you may want to consider hanging curtains to divide your space. I just redid my basement and I'm happy with the results https://www.homeroad.net/2021/09/basement-makeover-ideas-on-budget.html