I posted already on this subject:
The Zentangle Club leader has online class pre-registration. There is nothing in it that involves the Rec Center, it is all done through the club itself. And, if you are taking classes that are not full to capacity you don't need this service, but to the dozens of classes that are full to bursting this per-registration would be the answer to waiting in line.
I'll even give you the software they use:
Free online SignUp sheets, volunteer scheduling software , and volunteer management software. | SignUp.com
Hopefully, one or more of the leaders of the over full classes will see this post and check it out.