I've been to many different parts of Mexico. The best thing about places like Cozumel is going to be the shopping. Don't buy anything at stated price; instead, haggle. They absolutely love it. Some are better at it than others, and they almost look forward to it.

Most "Americanized" locations have purified water for you to drink. I don't know if I'd drink out of the tap. I wouldn't drink the coffee at a McDonalds. You will also find great deals on tacos and beer. I have seen 3 tacos and a beer for $5. More American money is traded in TJ than Mexican currency. Watch out for little kids trying to sell you stuff, I had one follow me around for a while in TJ. It will help if you are with your wife, restaurants/bars will give her a deal to get you to bring your wallet in. Electronic bulls are fun, too.

Enjoy!