Spray bottles are a great way of strengthening hand and finger muscles. Vertical surfaces are great for improving shoulder stability for pencil tasks. Why not combine them together?
I pegged up some paper to our fence, added some water based paint to some water in a cheap water bottle, and let him go for it! He took great pleasure in making sure the whole paper was covered.
I have also done this activity with older children with pictures drawn on the paper and easel and asked them to spray particular parts of the picture. Great for eye hand coordination and following directions!