ways to exercise a horse?

You could try turning her out for a while. Even a few hours a day will be a good way to relax her and keep her exercising.

I do not know how much your broken bone can take - if you take her on a lunge cavesson and rein it might be easier on you, get her used to approaching and touching things that could be scary to a horse, I am thinking bins, (hide some treats in them!) cones, tarpaulins etc that she might meet out on a hack - flags, flapping plastic bags etc.

If you personally cannot hold her because of the broken bone, get someone else to do it with you - you can come up with the ideas that you think you would like the horse to get used to and you will be able to watch her reactions.

Can you keep her indoors at all - restricting her grazing to just a couple of hours a day, on the poorest grass you can find. You do not say how tiny her dinner is, but a half a scoop is enough while she is trying slim down. You can increase this once she is fitter of course. Bulk the feeding with a bit of hay. As for exercise I would warm her up as normal, about 10 minutes walking, 10 minutes, building up 5 minutes every day until she is trotting at least half an hour which can be broken into 3 ten minute stints of active trotting with 5 minutes walk in between. The idea is to allow her just time to recover her breathing and then start off again. Continue in this vein for about 2 weeks and then you can start doing some cantering. If you can include steep hills in your rides, these are very good for building up wind and back muscles and hind quarters and generally increase the fitness without having to hammer her legs! The slower you can go up them the better. It requires a lot of effort to actually walk up a hill. Hopefully you should see more weight coming off her if you exercise her in this way. She is a cob so she will never be super slim, but you will increase her fitness. You are right to want to get the weight off as laminitis is a real risk in smaller ponies and cobs. They seem to be able to live and work on fresh air! They would do far more exercise in the wild, having to forage.

Mix things up!

Poles, cones, small (literally inches high) fences to step or follow you over, show jump blocks (use to walk over - lay on side to make as small as possible, bend between them - can also use jump wings), put tarpaulins out and get her to walk across them etc. You could loose jump her over fences with cones or blocks under them (cones and blocks are very light so you could move them without too much damage to your healing)

Good thing about the above stuff is that you can do it on the ground - it'll be different and you could loose her or lunge her over them (blocks that are laying down and tarpaulins). If you were to lunge over them, don't worry about side reins or a Pessoa and lunge roller - let her be able to sniff at the objects.

If you get her to follow you through these then you're doing a bit of exercise yourself - just walk briskly (you may encourage her to trot for a few strides every now and again if she's trying to keep up with you.)

If you do these in the saddle (after on the ground) then you wont bore yourself with endless circles in the school

To loosen legs soak in warm water with a soft sponge run your hands up and down this will loosen and cool her legs which is what a riding warm up does!

You can turn a horse in circles by putting it's halter on and turn the lead rope in small circles, this will make the horse alert, if the horse turns it's listening and watching you.

If it would be ok with you for this one if you have an arena let the horse in and say walk, trot, canter. Even if the horse goes anywhere in the arena, as ling ss they complete the selected word they loosen legs, stat alert, and can move freely without the weight of a rider.

Make sure the horse is cooled after exercise.

Also I hope you recover nicely pain free from your broken bone;)

Get her a big exercise ball. She may be wondering what it is at first but in a couple of days, might even take a week, she will be playing with it non - stop and she will have a blast!

Everyone has had good suggestions but I wanted to add teaching her to drive or longe-rein. A lot of people use it to train their young horses before they ride, but I find it a good way to keep things interesting for the older horse too.

For something different you could try an exercise ball..although some horses love it and others will lose their life, though the simplest and most beneficial thing would be turn out if it's available.

You could toss her out to play some energy off in the pasture. Teach her tricks such as nodding, shaking her head 'no', giving hugs, bowing, etc. (Mental exercise.)

I've never heard an American call it 'trekking', I've only ever heard of hacking.. (Yes, I'm American)