Kathleen Hagan has one requirement for her workouts: They must be fun. When Ms. Hagan wanted to add a “fun factor” to her winter routine (because “indoor rowing is boring,” she says), she discovered dance. The class is a mix of beginners from 40 to 80 years old. Ms. Hagan says she started out dancing to slow songs but has now mastered full routines. “It’s very good for memory, because you have to remember all of the dance steps in a sequence. It’s also great balance training,” she says.