How to define different classes of offers for a gym or store?

A friend of mine has asked me to create a management system for his gym.

Initially, the requirements were minimal, since he only offers a single training bundle of 30 training sessions, but then it turned out that he is going to move to a bigger place soon and he will likely want to implement different solutions for his customers, such as group workouts, training classes and so on. Since we still don't know yet what are the offers that he would like to add, we decided to keep the requirements minimal. However, I would like the system to be able to accommodate those changes eventually. Could you suggest me any Object oriented solutions to this problem?