This isn't quite free, but very inexpensive.
Ingredients:
a couple inexpensive PVC pipes (all the same size, we used 1/2")
end and corner PVC pieces (elbows, t-pieces, end caps, whatever looks good)
1 connector to connect the garden hose to the PVC pipes
Cut the PVC pipes, either into a variety of sizes or all the same size. Drill holes into the PVC pipe. Use the tiniest drill bit you have and drill sparingly, too many holes and there won't be any water pressure (learned that through experience!). Be creative with the holes and know there's no right or wrong. I let the kids cut the pipes and do the drilling, with safety glasses and supervision.
Throw the kids outside with all the PVC gear and let them get creative. Half the fun is in the building and seeing how each creation works, the other half the fun is in the getting wet, wet, wet.
This is a great "there's nothing to do" activity for the kids.
Total cost was less than $20, with every item purchased being reusable.
Here's another with instructions.
