Ground work (of ANY type) can be totally rewarding...I almost like it better than riding.
We've done oodles of things along the lines of desensitizing. Balls, hoola-hoops, tarps, water hoses, sprayers, coats, rugs, obnoxious kid's toys of all sorts. This list is endless.
I enjoy going for a walk, just like you would walk your dog.
Watermelon & a good brushing is/was always interesting.
obstacle courses are fun, we've used everything from landscaping timbers to folding chairs, cones, bicycles, lawn mowers, lawn chairs...weaving in and out or over. I like these in particular because it reinforced the horses' attention on you and helped to desensitize at the same time.
The options are really limited by your imagination.
Don't forget simple tricks like bowing and kisses or even shaking his head yes and no!!!