Do a 20m circle at each side of the arena, for example, A,E,B + C on both reins at trot. This is a really good warm up :)
ThatNinjaHorse suggested you do transitions, a great idea, but not walk-canter, halt-canter transitions. These are more advanced transitions and take time to develop.
Rein backs are good to, when you are just starting just do a few steps at a time. The same with leg yeilding, do a few steps, reward him by letting him walk out and then do a few more steps :) Once you've got leg-yeilding down pat at the walk and the trot, you can start incorperating it into your spirals.
Complete 1 20m circle, then legyeild onto a 15m circle, then onto a 12m circle - but remember, the smaller the circle the more work it is for your horse.
Play games! If you have a buddy to ride with and poles (as is the ones that stick up toward the sky) play bending games, etc, go trail riding, have fun!