Stretches, backing up, turning in the forehand/haunches, belly lifts, etc. I used to work with a senior horse who had terrible arthritis and could only walk with no rider. Walking up and down hills also helps
It depends on the reason for the walking exercises, but I enjoy taking horse's out for a walk in hand like a dog. It's something different and brings a new element to the same routes. It also allows me to introduce new things to young horses, or gives more time to allow the horse to suss out things they aren't sure of. In the arena, you could also try walking out routes laid out by poles.
Mental exercise is also becoming increasingly more common as a way to tire out a stabled horse. Stable toys and games (usually involving food so diet may have to be adjusted accordingly) are a great way to prevent boredom and adverse behaviours.
When my mare could only walk after illness I did flexing exercises with her. I'd walk with her and do pressure release on both sides and ask her to bend at the poll etc.