You can get a day pass at the Grand Hyatt to use their rooftop gym and pool. It has been several years but I think it was around $30 (the website says $40 now). Very nice gym with a view of the east side of the airfield and nice locker room shower area to clean up afterward. During the middle of the day I was the only one in there.
https://www.hyatt.com/en-US/hotel/te...yatt-dfw/dfwgh