Prague always had a powerful reputation for the occult. The golem legend comes to mind, but alchemy and astrology especially, always occupied an important spot in the history of the city, especially under Rudolf II of the Holy Roman Empire, House Habsburg. Under his reign, people like Tycho Brahe and Johannes Kepler, but also John Dee and Edward Kelley were welcome court, so it's only natural that you'll find an alchemy museum in the city. And in one of the oldest building in the city, even, in the staré město (old town). The museum, in fact, is where an alchemy laboratory used to be. Now, the museum is almost completely reconstructed to look old, bit like Disneyland, but the guides make no secret of it, indicating what was actually found and preserved and what was remade, so it didn't feel like a scam.
The laboratory starts with a study room, complete with desk and all the classic items you'd expect from an alchemist room. A bookshelf full of volumes, weird stuffed specimen from all over the world, a globe, scrolls, beakers and jars. A chandelier is in the middle of the ceiling, channelling the energies of the four elements to the room below, where the magnum opus was being performed. To reach the room below, you have to twist a gargoyle in the library, which opens a secret passage that leads downstairs, where several rooms, each with a purpose, are found, since alchemy was popular with Rudolf II, but not with the pope, so he had to hide things.
Other than the main laboratory, with the classic alchemic paraphernalia, there are rooms for glass blowing (alchemists had to make everything by themselves, for the same secrecy reasons), to dry herbs, plus some passages that connected the laboratory to the castle and to areas outside the former city walls.
In the end, it's a nice and interesting way to spend 40 minutes. The guides don't try to go for the camp/mysterious way, they explain clearly and neutrally how alchemy eventually became chemistry, when it was clear you couldn't transmute lead into gold, and paint an interesting picture of the XVI-XVII Century Prague and its culture.
You can even buy some souvenir at the attached shop, of course!