I’ve been to both spinning and rpm. Love it and was addicted for a while. Some instructors try things like press ups on the bars which i don’t bother with, but the music and intervals/timed climbs etc are great. Be careful if they tell you to take all the resistence off and pedal as fast as you can – keep some resistence on.
I sometimes do two classes in one night – 1hr45mins of spinning indoors is much better on winter nights when its snowing/icy etc, and more appealing the the turbo. My climbing out of the saddle (on the road bike) came on tons just by adding in spinning to my training.
As above, take spds, towel, water and hrm if you have one.