Ground Work

Winter is coming soon, and I came up with the game plan for Comet exercising-wise (I know, some people do or don't work in the winter. I work lightly, once or twice a week.)
Until it gets to be late December or early January, I'll probably ride. December thru late February or early March, I'll do all ground work.

The thing is, I don't know good exercises to try.

Any good ideas for ground work?
He's an English horse, btw.