A "couple" of hours might be cutting it fine, but if you really want to leave the airport and go into "real person" land, get to the "Central Bus Depot" and take a bus to "Bath Road". There are a stack of busses doing this - they go through the tunnel under the runways and pop out via roundabouts into Bath Road. This is external public perimeter road with a stack of airport hotels and you can even find a smokey pub

if you go down the right side street. You can stand against the fence and look at aircraft doing their business. The busses from the Central Bus Depot are free. Beyond Bath Road you pay. Just say "Bath Road" as you get on the bus.