We did the San Marcos from Don's a couple weeks ago.
Wide brimmed hat and sunscreen. Way to attach sunglasses to your head. Anything not tied down or attached that doesn't float will get lost to the bottom of the river.
River shoes that tie/attach to your feet. Rocks are sharp.
No glass or styrofoam. A big mug with a lid is perfect. Pour beer or mixed drink into it from the cooler as needed. Ice cold bloody mary + cold water + 100 degree day = perfect.
Buy a small inflatable boat, and put your cooler in it. My Canyon 55 fits perfect in an Explorer 200 with a little extra room for another small bag with sunscreen, cigars, etc, etc. See pic.
Rope and bungies to tie everything together. A bag that's ok to get wet, that you can tie to the cooler boat.
There's a couple islands/sand bars about half way down that are good for lunch. Take sammich stuff and make sure to bring sammich maker.
Go early, take the shuttle again for a second trip.
Go pro is small and waterproof, does pics and video, and perfect for the river.