I'd drive to downtown Frankfurt by Regionalbahn train, stroll the "Museumsufer" Main riverside with all the museums and exhibitions and maybe end up at the old Markthalle for some snacks. You could still return early to the airport for some lounge break.

The Rhine river to the west is quite nice as well. On a nice day a little boatride might be an option too. If you feel like hiking you could walk up the Feldberg mountain and back as well (more for the sport-minded). Oberursel is within reach of the Metro-Stadtbahn.