A character that spends so much time out in the sun should have their skin tan, becoming darker and darker. I don't know how it would work, would you have to be fully nude to get a full body tan? Or keep it simple, and say if you have your shirt off, you get a full body tan.
How it would work: You spend a lot of time outside in the sun with a part of your body exposed. The longer you spend time in the sun provides a tick to your invisible body tan meter. When your body tan meter becomes full, your character's skin becomes a visibly shade darker and resets to start over again.
This really serves no other purpose than vanity but I think it would be easy to implement and it would be interesting to see how it turns out.