The Lion, the Witch, and the Wardrobe was published in 1950 and featured the lead characters traveling to a magical land through the titular wardrobe. The Magician's Nephew came somewhat later, in 1955, and instead had a set of rings: one set that would bring the wearer to a mystical forest filled with ponds, and another that would let the wearer travel through these ponds into other worlds. I think this is one of the first literary uses of travel between different worlds (as opposed to "weird faraway places" like Gulliver's Travels) where those worlds aren't various afterlives (e.g. Dante's Inferno).