Which membership types can I sell?
Anything that fits name + price + duration: off-peak, unlimited, couples, student, corporate, ladies-only timings, family add-ons. Durations are monthly, quarterly, half-yearly, yearly and lifetime, which covers the usual gym ladder of "1 month, 3 months, 6 months, 12 months".
Because each package is its own product, a price change for new members never changes what an existing member already bought.