There are dozens of exercises and manuevers to work through depending on the rider's levels.

Basics -

If you put colored tape or objects by various parts of the riding area (round pen or arena) you can use them for tons of landmark directions. Or if you work in an arena with dressage letters (although not always fun for littles to only work on letters). example - Walk from the green tape to the red tape (point A to point B). Work on walk, halt, changing directions. Trot/canter. two point, no stirrups, one handed, sit trot, rising trot. - Simon Says (Intructor says).

Red Light Green Light -

Stand in the center or end of ring - call out red or green light (they should try to get to you (or touch the rail past you) without being caught by you - it's free to go on green light, but must stop when red light called. - if independent riders, face away at green light, and 'catch' them at red light if they move. Can throw in 'punishments' if caught moving - five steps back, lose stirrups for two laps of two point/posting, etc.

Barrel or Pole Pattern

Doesn't matter if English or Western - a pattern is great for steering work, control, and balance. If you don't have barrels or rodeo poles - use cones (so easy to set up any course without bulky objects)

I warmed my littles up with stretching exercises

Dropped stirrups/reins and work on stretching arms forward/backward (reach for ears/tail); windmill arms; touch toes with hands (left-left, right-right, left-right, right-left); wiggle legs, ankles, toes; roll head [show how heavy the head is and how big of a role your eyes affect steering]; play Head, Shoulders, Knees, and Toes (point to body parts. Apply to horse (ears, mane, withers, tail etc.).

All the stretching gets the rider loose and less tension, better balance, seat sinking into saddle, shoulders rolled back and hips open, arms moving with horse.

'Grocery Shopping' - set out grooming brushes around the arena (on the rail or on barrels/jump standards, etc.) and give the rider a bucket - either they carry it with them or you carry it while they steer. They have to steer towards the item, halt, and pick up the item - put into basket, and walk on. If multiple independent riders, make it a race game.

For more advanced students -

Leap Frog for cantering - student in front canters until they are at the back of the group, then repeat.

Swap horses - ride 'your' horse for fifteen/ten minutes, then switch, ride for 10-15 minutes, and switch again (repeat). You could have them back on their original horse - discuss the differences, pros/cons, easy/difficult, whatever they noticed between horses.

Musical Stalls - set ground poles to form a stall for each horse. One less than the players. Play music and stop it randomly.

If you have advanced students and can ride with them (or trail ride)

Follow the leader - have them follow you then after five minutes, switch to the next rider as a leader and they get five minutes to do whatever.