Booking System

Hello,

I have been asked to add a booking system to my local gyms website.
Can anyone recommend the best one based on the following:

Cheap but reliable, perhaps a one off fee rather than a monthly fee?
Allows the admin (myself) to add different classes for different times of the day with a limited amount of people that can attend per session.
An option for the user (consumer) to select their class and pay online or have the ability to reserve a space.
A clean design.

Any suggestions are appreciated, thank you!