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!
Igloos are cool. What kid hasn't wanted to build an igloo? You can easily build an milk jug igloo in your classroom with some planning, lots of milk jugs and hot glue. This could be a fun winter craft for the kids when it's too cold to go out.