Recently, as I was sorting through a drum-sized tin of marbles (long story), I found a bunch of clear glass blobs that had been mixed in. I wanted to do something Halloweeny with them, so after a few false starts, I came up with this twist on the classic glass marble magnet craft.
Journey with me now into the lair of the all-seeing beasts!
Once the Mod Podge is dry, and the image looks clear on the rounded side, you can add a magnet to the back of each one with tacky glue. I would not recommend you do what I did, which is to cut up a free business-card-magnet, because the pieces only barely hold up a single sheet of paper. Try self-adhesive flexible magnet tape or a sheet you can cut to fit; it won't be very strong, but it's easy and kid-friendly. For super strength, get round ceramic magnets (but only if there are no toddlers in the house; they're a swallowing hazard. Kids, please don't eat magnets).