Take heart, my dressage trainer assured me that he would, given the choice, NEVER work in an arena: he’d do all his riding out on tracks and in fields. The horses think more forwards, which is a huge advantage.
Of course this all requires that you treat a hack as a real training exercise, as opposed to just good fittening and legwork for the horse, without many demands (which, I must admit, is usually my default setting.)
One of the most obvious is zig-zagging across the track (see this post for more details.) Of course you can also work on travers, renvers, contrabending, and so on.
You can use the horse’s enthusiasm (especially if you are out with others – herd instinct can work to your advantage) to develop lengthened strides, medium, and extended. You can even get quite technical and do, say, 10 strides of Big Trot (whichever of the above your horse can do at the moment!) 20 of working, a few collected, back to Big Trot, etc.
I have read about dressage riders who get the horse’s piaffe and passage started at the gallops, using the horse’s natural instinct to go with the others. They contain that forward energy, and then praise the horse. Obviously this needs to be done judiciously… a horse plunging, rearing and leaping isn’t quite the aim!
Look ahead for ‘markers’ such as weeds, posts etc to do transitions at, so that you can check how long your horse takes to respond, and see how accurate you can get it.
On a nice grassy stretch, do a walk to canter, do say 20 strides of canter, and then either a simple change or a flying change to the other leg, and repeat.
You can work on sending on and bringing back… often you’ll end up with a lot more horse coming forward into the contact than you will when working in an arena!
Those exercises above all concentrate on the horse in particular, but out hacking you can really work on your own position and core strength too.
I knew a rider who did all her hacking out at jockey-length, in two-point position, to really strengthen her legs.
Two-point in walk is tricky, you might need a neck strap for balance! But two-point in trot and canter is much easier, and well worth doing to ensure you won’t get out of puff at the first event of the year. You’d want to be able to do 5-6 minutes of two-point position, without a rest and without being exhausted by it, before going xc, I think. If your quads start burning, you’re doing it right!
You can also work through all yoga/pilates/Alexander Technique or whatever other things you do to ensure your body is aligned. There’s loads of time to instil new good habits of posture to overlay any bad habits. Other people hacking out with you can check that you are sitting dead-centre, etc.
Hacking doesn’t have to be boring, it can be a really good way of improving horse and rider.