Hotel services and accommodations vary so much in Mexico, especially for those who stay in less expensive places, that its best to anticipate possible shortages. When Im traveling from one hotel to another, I always carry my "survival kit" in my day pack or in a small sack in my suitcase. It consists, more or less, of the following:
Flashlight: for power failures and midnight trips to the bathroom.
Bug dope: cheap hotels rarely have screens or mosquito nets.
Soap: I always drop those tiny hotel bars down the drain.
Towel: a small one, just in case. I sometimes pack an extra cotton t-shirt for a towel. It also doubles as a clean pillowcase and in emergencies can be worn as a shirt.
Toilet paper: always carry toilet paper.
Heating coil: this is a little device that plugs into the wall and heats up a cup of water for tea, instant coffee or bouillon. I wake up very early, before most cafés open, and the heating coil saves me from coffee withdrawal.
Tea, instant coffee, bouillon, Postum, etc.: saves money if you make it yourself. Nice for times when you cant or dont want to leave your room.
Cups: cheap hotels rarely provide drinking glasses or cups.
Drink: A liter of water is a must, so buy your own. Dont expect purified water in cheap hotels.
A small high-intensity reading lamp with a long cord or a 100-watt bulb to replace the hotels low-light bulb.
Good book: for bad nights.