I built a room I call 'the den'. Three walls are nearly entirely windows, looking out over my yard and large field. It has a wood burning stove to keep it extra warm, a nice stereo to help set the mood and a nice chair for reading.
(Actually the full name is 'the bear den' because one time family of black bears broke into it. But that is another story...)
I like my garage. I've turned it into a "dirty kitchen" where I can cook without stinking up the indoors kitchen (and the rest of the house). Also, I can keep all my cooking gadgets outside in my "dirty kitchen", and not clutter up the indoors kitchen.
When I've put on too much weight from all the food I cook in the garage, I can get on my treadmill, lift weights, ride my exercise bike, use my stair stepper, or stretch on my massage table. Yes, my garage also serves as a gym!