If the child is old enough, I suggest an introduction to watercolor paints. Done outside with a plastic cafe tray holding the media, messes can be easily avoide

If the child is old enough, I suggest an introduction to watercolor paints. Done outside with a plastic cafe tray holding the media, messes can be easily avoided, and the process can be guided by using coloring books(some of which come in paper heavy enough to take water-based pigments). The sound and tactile feeling of whipping a brush through the water cup and then applying it to the paper and seeing things happen is an extremely satisfying, low-waste, low-risk way of playing with water. It's not exactly the same as experiencing flowing water but it provides a similar amount of sensory stimulus.