I can imagine the body paint story ended badly... No need to look up the facts with an introduction like that.

Wasn't there also a Russian RTG core that was so hot it would melt the snow around it? Some scavengers found it, and got immediately blasted with a lethal does of radiation—as you would expect.

With this post, OP was clearly aiming for a minor annoyance or a frustrating little prank, but that story just gave me an idea that goes a fair bit beyond that... More like diabolical malice, but here goes anyway.

Sending one of those plutonium cores back in time to the neanderthals would be a pretty good candidate too. It doesn't really glow, which is a bummer, but it has other "magical" properties to compensate. The heat might still attract them to it, and the intense radiation would kill them within a day or two. If they somehow manage to touch the plutonium itself—a feat worthy of recognition—they could also experience its toxicity.