I think a good way around the issue would be to provide the option of sessions. So instead of €20 week per person you might be able to buy 10 or 20 sessions (at €3-4/session) which could be used by the whole family at anytime.
Not a resolution for the gym fanatics who want to go everyday but certainly would work for people on holiday for 3-4 weeks who maybe wanted to go into the gym a couple of times a week.