What do you use the suntan lotion for on wimpy kid boardwalk poptropica?

You don't use suntan lotion in Wimpy Kid: Boardwalk Poptropica. It's a game based on the Wimpy Kid books, and there's no sunbathing or beach-related activities that would require suntan lotion.

Perhaps you're thinking of a different game or a different part of the Wimpy Kid franchise? If you can remember more details, I might be able to help you further!