Camera Obscura is one of Edinburgh's oldest visitor attractions, using a 19th-century optical system to project a live panorama of the city onto a dish inside a darkened room. In bright weather the effect is sharp enough to feel almost magical, but the tower is still fun on grey days.
The rest of the building is given over to optical illusions, mirror tricks, and hands-on exhibits. It is touristy, but in the right way: central, playful, and especially good if you are travelling with children or want a break from heavier history.