Clothing/Shoes/Weather Gear: If you plan on going to Monterrey, no matter the time or place, you should always layer. I personally always carry a sweater, because during the summer they turn up the air-conditioning (clima) pretty high. If it's winter, layering is also recommended. Regios tend to get very cold, with heat on at pretty extreme temperatures when the weather outside is chilly.
Toiletries and Medical Supplies:
If you are thinking tasting some hot and powered dishes, like tacos with hot sauce, I recommend you bring some 'diarrea' medicine, just in case.
Miscellaneous: If you come in Summer, you must need a cap, and maybe some sunblock!!!
Light clotes, Monterrey is Really HOT, from April to October.
Toiletries and Medical Supplies: Bring At least Stomach Medicines, mostrly if you have never tried mexican food, is very spicy, and contains lot of fat.
Photo Equipment: All kind of batteries are available, and 120 V power.
Luggage and bags:
This is a totally personal decision. What kind of bag would YOU take anywhere?
Clothing/Shoes/Weather Gear: Wear light weight and light colored clothing. It is hot, hot, hot from April until November. Then it's just warm until March.
Toiletries and Medical Supplies: The normal stuff, but don't forget something for the tummy if you are delicate.
Photo Equipment: Monterrey is a completely modern town. Anything you don't have, you would be able to get.
Camping/Beach/Outdoor Gear: My idea of camping is no room service. There is no beach here, it's an industrial city. But there is a national park within the mountain range at the edge of the city. I'm sure there is a lot to do.
Miscellaneous: You should be sensitive enough to the culture to at least bring a Spanish phrase book if you don't speak some Spanish.
Clothing/Shoes/Weather Gear: The weather in Monterrey is quite extreme, so try to consider this when packing. It´s either very hot or very cold, and the conditions can change rapidly from one day to another.
Miscellaneous: Take some Pesos as often you must pay to use public restrooms, especially at gas stations. Usually the fee is a peso or two.