Madrid Fly is the biggest wind tunnel in Europe, which provides the experience of a free fall, without (thank God!) actually having to jump off a plane. It's 4.6 meters wide and 17 meters high, yet with a little preparation and proper guidance, it's absolutely safe for every person above the age of five.
The facilities around are specially designed for leisure activities, so if you're just bound to accompany a brave flyer, you can grab a cocktail and enjoy it in the garden or terrace, until they finish doing what they're doing.