I found “Glow in the dark garden balls” with an online search and bought one for my sister as a Christmas gift last year. During the day small flecks of fluorescent material absorb sunlight and at night they softly glow.
There is no backup battery needed, and no tricky set up. You either buy a mounting stake for it to sit on or hang it from a tree branch. These are hand blown glass balls that are an ethereal blue green, as they gently glow in your garden.
I haven’t seen it at night since my sister lives in another state, but I think it would be very mystical to see one glowing in the dark and may buy one for me this year. One online company that sells them is Signals, which is where I bought the one for my sister.