Now what kid does not want to be a pilot when he grows up? For all the future pilots and wannabes the Museo del Aire would be a delightful treat. Although it is a bit tricky to reach via transport, the trip is worth the effort: over 100 aircrafts, from the first bi-planes to gliders and helicopters, plus numerous examples of engines, missiles, and a large collection of wooden propellers.
If you can go to the museum on the first Sunday of the month (except January and August), you can actually witness those aircrafts fly. The museum itself is free of charge (but don’t forget to bring a valid ID – you’d have to leave it upon entering); however, if you are visiting the exhibition you’d have to make a donation of 6 euros to Fundación Infante de Orleans which organizes it.
Metro: Cuatro Vientos (line 10, C-5), then bus "Blas y Cia" which stops in front of the museum's entrance.