Lost my post again the other nite, so trying again now.
You could do some "trail driving" - even if it meant right on your own property. Vicki's and our places both have lots of trees - so we've set up trees to do different things with. We start in hand - teaching the ponies to both step over one leg at a time and straddle it and to jump it on command. Once your horse is really good at stopping and moving only one or two steps at a time, you could teach him how to move over logs with the cart. We've been on a couple of drives at trail riding places where we have had to drive our ponies over downed trees, while we were outside the cart - lifting the cart from the rear and rolling it up and over the bigger logs. We've always done it together so that there was little chance of a runaway since our ponies at that time were green and sometimes spooky, too. BUT they got good at doing it while we were driving regularly.
Below is Cupid working thru an obstacle w/ different sized logs and a blue tarp while pulling the pvc "shafts" and being asked to step over one leg at a time (didn't show the pics of him straddling the log that the black pipe is still resting on).
Crossing water is good, too. You can start with a tarp, then graduate to setting the tarp in a wood frame OR using a kiddie pool (put sand on the bottom to keep it from being too slick). Our carts and our weights would crush the kiddie pool, so we don't drive hooked thru that. BUT we do work them in hand and ground drive them thru. Vicki and her hubby set up a water obstacle using their "baby cat" with a bucket - and when it's full, it's a little above knee height on our smaller shetlands (Ami is 36" @ withers last time I measured her). The water collects leaves and isn't easily drained, so it turns stinky and dark - the ponies really don't like crossing it - it goes against their wishes. We just keep working with them. We get some really interesting shots at times when they try avoiding the water!! In one, it looked like the pony was "walking on water", LOL.
After Oly was done splashing thru the obstacle, it was almost empty of water. But where it was on the tarp, it ran back in somewhat. This spring/early summer - it was completely dry. Now with the rain again, it's a lot fuller than in any of the above photos. Here's one of Cupid ground driving while pulling the pvc...
We have ponies that work thru water a lot that will still sometimes look at new water set ups and snort and try to work around it. I so want to do the ocean drive one of these years - either with a single or with a pair of ponies to the wagon...
You could also look thru the book "Arena Exercises" or the one for "lounging and long reining" - both by Cherry Wilson (I think). Lots of patterns and ideas - I think over 100 in each book. Some from the first one would be hard to do while ground driving but a lot fo them could be done while hitched (written for ridden exercises). The 2nd book is made for both ground driving and hitched driving. Mine is packed at the moment - I always forget what's in it for exercises.
Set up a bridge that is heavy and wide enuff to drive over. Practice walking over it first - then add different gaits - both before, over it and after it. Lots of videos on YouTube of stuff like that! Fun to watch, too.
Set up things to drive past - sometimes just the push in posts with Wal-Mart plastic bags blowing in the breeze is a good one. Once your horse/you bored with that - set the posts up to weave thru and do so at different speeds.
Could drive between hay bales, use them as weaves, use them to figure 8 around. Teach your horse to calmly canter/gallop while pulling your cart - then do the obstacles w/ different speeds between them. Always changing it up and not allowing him to pick the speed.
Work towards both speed and precision, good driving. When you can get both - you will have a well trained horse.
At one of the trail riding venues, I was able to do a controlled gallop with a 3 abreast hitch to our forecart w/ 2 foals along with us too. It looked like a 5 abreast hitch until the two fillies peeled away to go check out the church group in the picnic area before we got to our trailer (food and petting, O YEA, you could see their brains clicking!!). YES, I had galloped towards the trailer hoping to keep the two fillies with me rather than having them in amongst folks who may not have known how to deal with loose babies. I still had to go back and collect them up - I wasn't expecting the group or I would have caught them before we came out into the camping area again... Totally wasn't expecting all the people, LOL. No one there when I arrived, hitched and left for a 1.5 hour drive. When I caught up the fillies - two of the folks feeding them hot dog buns commented that it was just like a chariot scene from Ben Hurr - which I hadn't even thought of.
Those two fillies are two this year and will start some basic harness work this next winter...