A product doesn’t come much simpler than this: One round electronics board, one LED and one battery in one tiny pack.
Stick it under the roof of your model train coach, in a dolls house or anywhere where there is little space and you don't want to hook up with wires, wave the magnetic wand close to it and you have light!
Size: 11x11x5 mm.
Battery life is about 40 hours. CR 927 batteries are easy to replace and cheap, available here on eBay.