Bit too short to go into town. Figure 1h15 to get out of the airport and two 30mins taxi rides, and wanting to be back at the airport 1h before. That leaves you with 1h45. Presuming this is between LGW-MAD-LGW it gives you from 14:15 to 18:45 or so depending on the day and current timings. I suppose you could have a lunch in town and come back...
Otherwise the Plenilunio near the airport can do