It might be interesting to have sunscreen that comes in “fun” colors (maybe pict blue or Orion slave girl green) that fades as it’s exposed to UV light, ideally fading away completely at about the point where it no longer provides adequate protection from UV rays.

People could even get different colors of sunscreen and paint designs with them—like temporary tattoos, only functional. You could go to the beach and get a full bodypaint airbrush to protect you from the sun and fade away in a couple of hours.

It might be more fun, albeit less useful, to have sunscreen that starts transparent but becomes, say, hot pink as it approaches the maximum UV protection. (Less useful because it could be washed off by swimming and you might not realize you’re being burnt.)