UV lights, also known as black lights, are fun, too. There are all kinds of things around visible to only UV light. Currency, Led Zeppelin posters, public bathrooms and motel rooms, just to name a few.
i just recently ordered a few of these from ledshoppe just because. there are all sorts of things visible under UV/blacklight. I live in new york, and when your license is exposed to UV, it completely changes. i know certain types of passports and money do the same.
What off the top of my head, what i know changes under UV:
aged bananas change under UV (the brown spots)
certain bodily fluids (HEAVY amounts of skin oil, like on video game controllers or remotes and phones or keyboards, but not [usually] things like knives or windows. idk about blood havent tested it )
some types of animals (Ive heard scorpions) and fish
heavy dirt/bacteria spots..you can use it to make sure your glassware/hands etc is literally SPOTLESS
anything with phosphur.
Im sure theres more, but those are the ones i can think of.