Toiletries and Medical Supplies: We encountered some problems with fleas in Sukhothai - which we were subsequently told is a problem during the dryer weather there. So, if you have a repellent which covers these little bities, as well as the usual mosquitoes, its ideal.
Luggage and bags:
There is not much shopping to be done, so additional bags, or the bag in the bag situation will not be required...reserve that for trip to jatujak in bangkok.
Clothing/Shoes/Weather Gear: Light cotton t-shirt and pants/skirts.
You'd be walking on grass quite a bit. If you dont like the feeling of dry grass in your sandals, bring a pair of walking or sports shoes for the walkabout around the ruins....and oh, a hat with large brims will come in handy in the afternoons.
Toiletries and Medical Supplies: Mosquito repellant is mandatory, depending on the time of the year and level of tolerance, sun tan lotion.
January is supposed to be 'winter' at that altitude, but it was hot, even for tropical me.
Photo Equipment: There is a camera shop at the Big C half way between the old and new town for your camera needs. But do load up on the batteries before going to the sites. It will be frustrating when you are busy snapping away and the camera goes buzz buzz due to low bat...and snapping away you shall.
Miscellaneous: Adaptor for your phone and laptops...and whatever else your electrical needs.