Here is jsut a few ideas to get going. Keep in mind that anything too big needs to be ok'd usually by management, I once had to buy a pair of sneakers in Cabo just to go ATV riding. I bought something that I knew I wouldnt use again and left them for the maid but needed to get permission from the management, so they didnt think she was stealing them.
Better, travel sized lotions, like the kind you get from the malls
Nice little Soaps
Nail polish in smaller bottles
Try to think of things that would be a nice touch, Remember they live compleatly different from us and occasionally its nice to have somethign that you can not get where you live.
We also in the past have taken down flip flops and barterd with them on the beach. They can be expensive in Mexico.