Born and raised in Sheboygan. I think you are required to dislike the place you grow up. But now that I don't live there I realize how much the city had to offer. The lakefront is gorgeous. The art scene is not bad either for a city that size.
GLENSWORLD did a pretty good job of covering everything I'd like to cover. My personal favorites are John Michael Kohler Arts Center (free), trails at Terre Andre State Park (expecially the boardwalk through the dunes), walking out to the lighthouse, the riverfront. Also head out to Kohler and visit the Kohler Design Center (free). If you have the money, stay at the American Club. If you don't have the money (like me) have some three cheese soup at the Horse & Plow or ice cream at The Greenhouse and eat it in the beautiful courtyard. Both of these are in the American Club. If you are looking for theater, the Sheboygan Theater Company is an excellent community theater.