Thank to all of us for your good tips !
Flexing its neck and a deep seat
I agree with Maddy but also be careful because it's not your regular environment ! ;)
You can do everything you do usually !! :)
Walk trot transitions are perfect, try to be as smooth as possible and don't lose the forward energy either going up or down.
Lateral work has done wonders for my horse, exercises such as leg yield, haunches in, shoulder in, etc.
But always have forward movement, think impulsion, not speed. Also try lots of transitions, working from walk to halt, walk-trot-halt, etc.