Here are the stretching exercises we do - I use carrots/packer pellets:
1. Stand toward your horse's flank and get him to turn his head to where his nose touches near his belly. Repeat on both sides.
2. Place carrot/treat between his legs and get him to lower his head.
3. Stand on a mounting block (for my horse it's a 55 gallon drum!), hold treat up in the air so he has to really elongate his neck to reach to get it.
4. Pick up front hoof like you are going to clean it but instead gently pull it forward, same with the back.
As far as fun exercises while riding, I got one of those huge horse soccer balls and have had Solon do figure eight's while pushing it. It's really fun, but really tough!!
I also put obstacles out and have him work over/around them. Poles on the ground, hay bales, tarps.
One I do on the ground is to actually play kick ball with him. So I roll the ball to him and then he rolls it back but he also has to try and move it in a pattern so it's not just a straight back and forth line.
Hope some of these help!
Unless it weighs a ton... it's just a horse. Draft horse motto.