Long reign , circles , bending , ground poles, lead changes etc, basically the basics depending on what your gonna do that ride !
Its good to do a bit on a long rein when you first start riding as it encourage the horse to stretch down and hold themselves which stretches and strengthens the long muscles in the back. Lots of circles get them flexible, thinking and listening. Any gentle pole work, particularly before jumping, is great for getting them to pick up their feet , look down, stretch their stride and think as well. This all helps with topline as well as warming up nicely.
Nicola Smith at Dressage Rider Training is often posting on Facebook ideas of exercises to help with riding.
She actually has a program you can sign up to, but also offers these freebies. Take a look there. Lots to do with hip opening and movement and flexibility before you get on to help with position.