I'm in ZRH at the moment for work. A little gift bag was given to us which included a free visit to the Viewing gallery at the airport here.
All I can say is it's brilliant. The Swiss really know how to make things work. They only charge about 5CHF for adults and less for children so it's not exactly bank breaking.I think it's free if you buy something from the fairly well stocked aviation themed gift shop as you enter. When you're there you have the choice of a restaurant, gift shop, and a small cafe. It's all open air, but has plenty of cover for if it rains. A play area keeps younger kids amused and the views of the runways and ramp area are just great. They have various interactive things, such as a screen above each stand which shows aircraft specs and the flight that's boarding from there. They have an area you can hear the air traffic control and live video telescopes which highlight various parts of the ramp and point out the odd flight with little arrows as they move around the airfield.
I see no reason at all why more airports can't offer the same thing as ZRH. They make money from it so LHR should be interested in that. As it's so good and so close to all the action and so cheap it attracts enthusiasts and because you have to clear security to get in it's secure too. They've turned going to the airport into a day trip, like it used to be.
Also, ZRH offers bus tours of the ramp area too. I believe the commentary is all German and it only operates on certain days, but at 8CHF it's also a pretty good deal. I have a free pass for this too, but won't be able to make use of it as there's no tours when I'm free!
