I first saw this over at The Mommies Made Me Do It, and have since seen lots of others. I knew it was something we had to try. It couldn't be any cheaper either. We picked up a plastic, paint tarp at Walmart for .98. We had duct tape and the water from the hose already. So basically, this cost less than 1.00 to do. We did this back in April. It was a nice, warm day, but not warm enough to get soaked. So, this was perfect.
To make the bag, simply fold the tarp in half, duct tape the ends together and leave a smallish opening for the hose to go in. After you fill it with water, simply tape the opening closed with more duct tape.
Try to find a flat spot in your yard, otherwise the bag will start moving downhill!
We actually filled this up in the front yard, emptied it and refilled it in the back yard the next day. It did get a few leaks, but duct tape is a cure all in this case. It is also super fun to get the top of the bag wet, makes for a giant slip n slide.
I went ahead and bought a couple of these tarps to use over the summer and every time, my kids have a blast. We tried putting some object in it, but they just ended up huddled together on one side of the bag. I would love to hear about your bags, if you make one.