I think the best exercises in dressage that I still use everyday to warm up my horse before I start with the pirouettes or half passes are :
- Zig zag Leg yielding in Walk ( zig zag is better then a normal leg yield because your horse never knows where you want to go and needs to really follow your leg ) best way to do it is to start on the centerline
- in trot smalll circles 8-10m for the balance and lateral work sometimes I ride an 8 also 10 m
-in Walk 5 steps backwards , 5 step forwards , 5 steps backwards etc. make sure your horse doesn't rest a second between the transition from forward to backward and so on..( it is an old german exercise and it feels awful but makes your horse really soft and fast in the reactions
-3- 5 Loop Serpentines in different ways:
1. Way in trot : trot and bend in every serpentine and a soon as you leave the outside track to change direction do a few steps medium trot over the centerline and then come back to normal trot and bend again for the next serpentine. ( this is difficult and doesn't feel great but it is really good to improve your horses trot)
2. way in trot : trot and bend and everytime you pass the centerline transition to walk ( then 5 steps walk) and then trot and bend for the next serpentine.
3. way in canter: same like the second method but with canter / walk / canter transitions.
4. way in canter ( advanced ) everytime you cross the centerline flying change.
A good stirrup less workout can only do you good in dressage! Just make sure not to use your hands to balance.