Watch the film "American Beauty" for reference (but not the end). A garage is the ideal spot for a man cave. Clear out a corner and lay down a cheap, probably awesomely hideous rug. Perhaps add a weight bench and a couple of ratty couches and chairs you have kept since your first college apartment. Add an old TV (that gets satellite, of course) and a hand-me-down sound system and you're set. A fooseball table, ping pong table or even a pool table might be in the cards here. Think about installing a fridge too, just to completely separate yourself from the nosy comforts of the neighboring house you live in. The perfect home away from home for any guy.
As garages equate to slightly grimy messy spaces and art studios to beautiful messy spaces, the two match together well. Tarps can be laid on the floor to lay canvases on top of, so that you can paint Pollack-style. Easels can be set up against the walls and paintings can be strewn about and stored easily. A utility sink, which is rather easy to install, if there's no plumbing already, should probably be added to wash all the paintbrushes and other tools regularly. Yet this addition to the garage could become the perfect space to concentrate and create.
Remember when you were going to be in a rock band and tour the country and be on the covers of magazines? And while you know it's probably never going to happen, a good jam session with some of your old buddies is as fun as anything and great way to release stress or tension or just relax for a few hours. So in that garage addition, make space for a drum set and some amps. Staple-gun foam, cardboard and egg cartons to the walls, and throw down some old or cheap carpets to insulate the sound and not rock the neighbors.