A small exhibition space on Grandi devoted entirely to the aurora borealis: the physics of solar wind hitting the magnetic field, the folklore (Vikings thought the lights were the reflections of dead warriors on their armour), and a darkened cinema room with a thirty-minute film of professionally shot displays projected wraparound on a high-definition screen.
For anyone who comes to Iceland in summer and so cannot see the lights in person, it is the next best thing. Open year-round; allow an hour.