What can i do to reclaim wasted space in our tiny yard?

If you think of a box when you come out of the house into the back yard, to the opposite end and dead center is a stone fireplace that's been there forever. we do use it and enjoy it. to the right and coming up from the back there is maple tree, then the garage door and then the gate to the driveway. On the left is our daughter's swing set. there is some open green space in the middle that the kids use to play and run around. off the house is a deck and patio. back on the left there is an extra strip that goes back maybe 30 feet and is probably 10 or 12 feet deep. towards the front of that space is a big tree (over 100 years old) that we would never cut, but the roots make it impossible to grow anything. and because it is "around the corner" it never gets used. any suggestions on what we could do to make it functioning usable space?