Actually one place I work at had an excellent way of promoting the spinning class recently. The program has been in place for a while… so the promotion was used to re-invigorate interest and it worked.
There is a small fee for classes at this club. So, for one month, for every 30 miles ridden during classes, a person would get a free class voucher. On top of that the person with the top mileage got a “yellow” jersey. There were some other prizes as well and it really got people excited.
While the spin classes take part all day, the attendance in all classes went up. So, create a program that involves a bit of competition and specific rewards. In the above program, everyone got a free class for the mileage, while the hard core competitive types got a “trophy.” Everybody won something.