Well as for bowing and lying down, it does depend on which leg is injured. If it was a front leg, I might be more hesitant to teach bowing, but as long as he can get down alright in the pasture to roll/sleep I don't see the harm in trying out these tricks. You could probably tell from his body language if you are hurting him!
There are of course tons of other fun tricks and activities you can do without getting on his back. :) The key is to be creative! You could set up little trail courses with bridges, ground poles in interesting patterns for him to walk/trot over or back through, tarps to walk over, raincoats to move over his back as if you were riding him and putting it on, and other obstacles! You could introduce him to new "scary" things and do some exposure work. If you have local trails or a horse-friendly neighborhood, you could try taking him on a "leading" trail - like a little trail ride adventure, except you would walk next to him instead of on his back! There are lots of fun ideas out there for your pony!
As for treats, no one here can suggest the best treat for your horse. The best way to figure that out is through trial-and-error. Gather up some things he might find tasty and see what his favourites are! It doesn't have to be sweet for him to think it's yummy. You can try carrots and apples, but also pears, grapes, crumbled granola bars, watermelon, non-sugary cereals, store-bought horse treats... he might even enjoy healthy people-snacks like sunchips!
Keep in mind, however, that you don't want to over-treat him. With most of these treats he won't get sick, but he could end up spoiled and pushy!