You're going to run out of things to do in any one place, unless you're in a pretty touristy place (and unless you are into fishing or golf). You could easily spend 4 days in a place like Cabo. -maybe even Acapulco. Otherwise, I suggest you fly into a place like D.F. (mexico city) and rent a car. Cruising around the highlands south of the city staying in places like Taxco, Cuernavaca, and random old castles that Cortez's kids built is fun! There's even a Zoofari to cruise through down there.
Know what...maybe that's too much stuff to do. I guess if you're flying in the first day, spend the next two days drinking pina coladas on the beach and shopping, and fly out on the fourth...yeah, that's plenty.
Ok, forget everything above and just pick a place on the beach with decently priced plane tickets! Cabo is the most 'Americanized' of them. -I would guess the safest too.