Class booking with real capacity is the heart of it. If the six o'clock class has twelve spaces, twelve people should be able to book and the thirteenth should go on a waitlist — otherwise somebody drives over at quarter to six for nothing.
Collecting the money is the work nobody sees. Direct debits, freezes, failed payments and notice periods tracked automatically save several hours a month and prevent the awkward conversations that come from noticing three months late.
Someone who hasn't been in for three weeks is about to cancel. That figure is the real early-warning system in a box, and it isn't in any diary — it comes out of attendance, which is why attendance is worth recording at all.