Speaking of not drinking the water: when you have a frozen mixed drink or anything else over ice, remember the ice was made from the water. And as it melts into your glass, it comes looking for you
Buy tequila, vanilla, large denomination Mexican coins, and Cuban cigars.
Tequila is muy cheap, and even if you exceed your limit (2 liters I think) it's still worth paying the extra tax on it. It's about 1/4 of the cost in Ohio's government run liquor stores. So even paying taxes on it, it will still cost much less. And it keeps forever.
If you are into cooking (and even if you aren't) a bottle of pure vanilla is a good investment. Here a bottle is about four ounces. There a bottle is sold either in liters or 750 ml. Here that four ounce bottle costs about $1.98. There the one-liter bottle costs about $3.98 (American). Again, it will keep forever - well, it seems like it.
Mexican coins are made of pure silver, unlike our copper or zinc filled dimes, quarters, and halves. If you hit the beaches, a bunch of folks will try to get you to buy various and sundry items. Go for the coins, but make sure they are real. Even if you aren't into the silver aspect, they look and sound pretty cool. By sound, I mean they *ching* like our money used to.
If you are into cigars at all, try a couple or so good Cuban cigars. They are legal in Mexico. Make sure you smoke them before you try to cross back up here though. ...mmmm...Havanas...