We went down to New Braunfels for tubing today. We were a total of 11 people and 3 coolers (so 14 tubes). You basically sit on a rubber tire for 4 hours or so while drinking beer (or any other liquid you bring). Usually you get horribly sunburned, scarred and disfigured for life; but a good combination of industrial strength sunscreen lotion (did you ever see RoboCop?) and clouds saved me.
Some advise for future floaters: avoid banging your head with the rocks!