Redditor EffinD created this after finding that sports left him sweaty and uncomfortable with no shower nearby. All you need is an empty container, a bottle of shower gel, some water and a freezer. Scroll down to see the steps, or go here to see the original how-to.
First, fill the container about a third of the way up with water and squeeze in some of the soap.
Stir it up so that the soap and water have mixed together evenly.
Throw the container in the freezer. You’ll want to freeze it all the way into a block of ice before you take it out.
Take it out of the freezer and add another small layer of water (without soap this time). Make sure to leave room for the ice to expand; you don’t want to fill it all the way up to the top. Throw it back in the freezer.
Put the container in a cooler or freezer when you aren’t using it, and when you want to rinse, you can pop the soapy ice block out of the container. The larger portion is soapy ice, while the top layer is just clear water.
You can then wash yourself off with the soapy side and rinse the soap off with the clear side. Hit the link for the original how-to.