J.L.
When my son was potty training, I colored water blue and red with food coloring and then froze it in ice trays. After they were frozen I would store them in a ziploc freezer bag (in the freezer). Anytime he would need to go, he would ask for a cube. He had great time aiming for the cube and seeing what color the water would change to.
I've already seen it suggested but Cheerios would work well too. We have also used the biodegradable packaging peanuts that dissolve in water.
Good luck,
J.