Hey! The thing about a roundpen is... there isn't a lot you can do, but go in circles.... unfortunately.
The boring stuff is often the most important stuff anyway. Make sure you have completed the inital stages of roundpenning first before changing it up. Consistency is how a horse learns, and variation is what keeps it interesting; but you have to have enough consistency for the horse to learn before adding variation.
Once you have your horse going well, then you could change it up by moving outside the roundpen and working your horse over logs, ditches, on the side of a hill, etc., somewhere your horse has to concentrate on where he is putting his feet. Give him a purpose for the groundwork.
Roundpens shut horses down mentally because of their size. So, you want to stay in there long enough to get done what needs to be done, but then get out of there so you can free up the horse's feet and mind.