Gym spin classes are all about the instructor. The best I've seen are current road bike or track competitors. They need to work out and they make you work, too. I've always found doing hard efforts in good company to provide sufficient "interest."

After some years of spin classes, I quit going in favor of working out on my resistance rollers at home, with big fans and loud music. Rollers will definitely hold your attention. One slip in concentration and you're on the floor. Plus you're on your real bike, riding your real tires, and except for some details like not having much momentum, it feels a lot like the road. Resistance rollers because they allow you to do hard intervals, low cadence pedaling, all sorts of drills and efforts. I found this to be more effective out on the road than was spin class. I think these are the best currently available: https://www.sportcrafters.com/produc...ve-pro-rollers

I've been riding resistance rollers for over 20 years, haven't tired of them yet.