Take a 2 liter soda bottle, poke holes in it. Attach to a garden hose. Toss over a tree branch and let hang for a kids water sprinkler. You can loop it over a tree branch and let kids bat it around for more watery fun!
Easy kiddie kitchen sinks: find a table, cut two holes, insert plastic tubs - can't find this on the link but the picture pretty much tells you what to do! Great idea for outdoor play space In the summer
20+ Cool and Fun Water Play Ideas for Kids in Summer