since you have workout equipment in it why not turn it into a work out space.

I need to see the whole house layout. This appears to be basement space so I would want to know what is in the rest of the house.

Nobody can really answer. Is it a basement? Or you have a raised rnch? What is UP stairs? Before you can "furnish", you need to know the purpose of the space,How you will use it, or need to use it.

Text does not inform. What does? A scale drawing with every wall, window door passage and stair noted, and all the other information as well.

Not nearly enough context to your question,

This is a mud room with a slider. That is odd. Someone wanted access to the outside from this level in a dramatic way. Why? Is there a pool? What is in the back yard would influence what I offered in this room.

all of the above questions need clarification. Plus, what don't you have in the rest of the house? An office, reading space, craft room, more storage? What is the budget for creating? Will it be DIY? painting? flooring change? area rug? Do you need to reuse any/all of the furniture?

I dont think its that hard. first step rug, second step rethink the art and mirror. and light fixture.

Thanks for all the comments everyone - this gave me some clear ideas!