A gentle workout on a sunny beach or park is far more pleasant than sweating away in a gymnasium, and it acts as a social occasion, allowing people to chat and make friends. You could even take your group on jogging and cycling excursions, giving people the chance to explore the local countryside.
This idea is perfect for the summer months. Contact existing exercise classes and discuss your proposition - they may be interested in employing your services as an extension of their class.
Alternatively, go it alone. Advertise in shop windows, sports clubs, the local paper, tourist attractions and accommodation. Either yourself or a friend could hand out flyers to passing tourists and locals - make sure the times and dates of your classes is clearly printed on it. An attractive sign is always a good idea too.