I just got back from Mexico yesterday, had my surgery on the 28th with Dr. Kelly. For last minute items, I would recommend bringing some Gatorade powder packets rather than spending a fortune at the airport for it. If you booked your surgery through Trish then I would say you need nothing else as she will get you everything you need while in Mexico. Make sure to make copies of your driver's license, passport, and credit card and store them in your bags. I would also recommend packing light if your going to be moving your luggage after the surgery. I would also recommend taking some sweaters/sweat pants if you get easily get cold as it gets a little chilly at night in Tijuana. I took $250 dollars to Mexico with me and it covered an additional two nights I stayed at The Lucerna and some light souvenir shopping.
Hope your surgery goes well.
Take care,
Darren