For you or your horse?
For your horse, anything that gets them working in a proper frame is going to be the best way to develop their fitness. You can do gridwork and poles all you want, but if your horse isn't using themselves properly, it's not going to have any huge benefits. Transitions, bending, and encouraging them to push from their hind legs while being supple in the mouth will encourage them to stretch down over the topline and work themselves properly. Work with a coach or trainer to develop this.
For you, two-point is fantastic for developing balance, and leg strength. Sitting trot is great for balance, learning to move with your horse, and if done properly, engaging and strengthening your core. Stirrupless work is one of the best ways to gain an independent seat, balance, and leg strength. If you are committed to developing your own fitness as a rider (if that's what the question is asking), consider doing some sort of physical activity outside of riding 2-4x a week. Anything you enjoy will be beneficial. I enjoy powerlifting, but running, yoga, fitness classes, etc. will all help you become a stronger rider.
Anything that really engages their brain. Transitions, changes of direction, leg yield, side pass, backing up, turn on haunches/forehand, ground work, anything that makes them work their brain and really get thinking. Put a pole or jump in the middle of the arena, and see how many ways you can go over it! Sidepass over it, split it down the middle (having them walk straight over it length wise, keeping it between their legs), back up over it, do figure eights over it, all sorts of things! Lots of brain work is a great work idea, and if you're in a rush, they might not even break a sweat, so easy to put away!
I find grid work is amazing. After a quick warm up you can pop your horses over a variety of grids at whatever height you wish. It gets them working their muscles, helps the horse become quicker when jumping (I.e not hanging in the air) and teaches them to be careful with their feet