What is the best backyard water sprinkler for kids to play in?

I want to buy a present for my friends who have two kids. I think they’d love a sprinkler to play with in the yard during hot summer days. As a city dweller I’ve never had a back yard, much less a sprinkler. Anyone have a recommendation for one that I could buy on amazon that would be fun for kids? Doesn’t have to be designed for kids or have a kid theme.