Power rings

Power Rings appear in the episode, "My Two Favorite People." The ones Finn had, produced little sounds (one of them is from the PlayStation game "Tom & Jerry: in House Trap" ) and gave off projections of various tiny images such as fire, a pinwheel, a red planet, and a pillar of yellow smoke. When all brought together, Finn's power rings produced a large blue flame. Finn's rings were cursed and could not be taken off at the time but, Finn seemingly found a way to remove them. The Forest Wizard gave them out for free.

Given that there are Power Rings on so many fingers, for both Finn and Lady Rainicorn, this may be a reference to Mandarin's Rings, Mandarin being a nemesis of Iron Man. However, Finn's and Lady Rainicorn's rings are far less aggressive and more subtle than Mandarin's, probably because they are more like toys, if the Wizard would give them to anyone.