When you offer each type of class can be just as important as what you offer. After all, you may have a great group strength class, but it might not be well-attended if you offer it at 10 a.m.
The rule here is to find out when your members can and will attend a class. If you have a number of retired members, group classes catered to older adults can be successful in the middle of the day (and this may help bring in members during your slow hours).
Consider all of yourmembers: families, stay-at-home parents, students, seniors, etc..Figure out a schedule that works best for the majority of them. And, as always, get feedback. Getting more members involved in your group exercise program may be as simple as changing your schedule.