Depending on the level of the horse, this is mine:
Walk/trot around arena a few times, occasionally going across diagonal & sometimes doing serpentines when I remember, on a loose rein.
Stop in the middle of the arena to flex from side to side at stand still, turn on the forehand/haunches in both directions, back up, and side-pass.
Then go out on the rail and do circles asking the horse to soften at the poll and have body at a curve walk/trot, switching sides by doing a figure eight inside the circle.
After a few minutes I go out on the rail and canter around the arena doing a figure eight to change direction and practice flying changes. After a few minutes bumping it up to an extended canter to get the legs stretched a little more.