We are hoping next year at our show to offer ADT classes. I know there are a lot of pony people up this way starting to attend ADS events which have CDE's. The VSE classes are growing pretty fast for ADS. At a show we could probably only have the three driving type classes. Although I would like to try to figure out a CDE course for it, but if I remember correctly there has to be a water hazard. Most arenas won't allow you to dig up their base.
That's great that you want to offer some ADS-style classes!
Just to clarify terms a little bit an ADT is an "Arena Driving Trial" which is a one-day version of a CDE or "Combined Driving Event." A CDE is a particular type of stand-alone competition, not a class offered at another ADS ("American Driving Society") event. I'm not sure what you mean by the three driving type classes as there are no classes in a CDE, only phases.
A CDE has three phases where you earn penalty points that are added into your final score, determining the winner. There's dressage, the marathon (which is that course I think you're thinking of), and cones. Cones is an easy one to set up in an arena as all you need is tennis balls and up to twenty sets of cones to put them on. For each competitor you measure the distance from the outside of one wheel at ground level to the outside of the other wheel at ground level and then add a predetermined amount of clearance, then set all the cone pairs that far apart. The goal is for the competitor to make it through the numbered cones without knocking off any of the tennis balls. The person with the fewest penalty points from knocking balls down or going too slow would win in that class. In a real CDE the cones phase doesn't have a winner, it just adds to your overall penalty score the for final win.
A water hazard is not required for marathon and certainly not for the Arena Driving Trial version of one. Besides, you wouldn't be holding sanctioned classes anyway so the rules don't really matter for you!
Do whatever sounds fun.
On the original topic, if you can't get a trainer to work with you and your horse then I would strongly recommend buying Clay Maier's long-lining and "Starting Your Driving Horse" DVD's
http://www.claymaier.com/tutorials-drive.php and reading Heike Bean's "Carriage Driving: A Logical Approach through Dressage Training." I thoroughly approve of the common-sense and safety espoused in both approaches and both will take you from start to finish. Wagons are indeed more utilitarian around a farm but you may need a pair for that unless your mini is almost pony sized. Most minis aren't up to hauling a 4-wheeler up hills or holding it back going down one, especially if it's loaded. It's just too heavy!
While you are reading books or watching DVD's and getting ready to start, the best thing you can do is make sure your horse is used to just about anything scary you can think of and has a perfect "Whoa." Teach ground-tying to build patience and desensitize them to dogs, kids, waving trash bags, poles being dragged around them, sand-bags and tires on ropes, a plastic bag full of pop cans being clanked around their hind end, ropes all over their bodies and even under their tails and between their legs,
everything! You absolutely cannot overly desensitize a driving horse.
Leia