Nowadays in Thailand most people under the age of 40 wear western style clothing. And younger women often wear clothes as revealing as anything as you'd find in the west, often with a tattoo for effect. Shorts and T-shirt are quite acceptable for day to day missions and excursions. Try to keep it at that though.
Men not wearing a shirt around Chiang Mai will be considered impolite and some tourists have even been harassed by police to put their shirt on. It' simple: Thais don't like it.
Thais are quick to judge a person by the clothes they wear. For visits to immigration or official locations you are wise to dress politely. That translates as long pants and a decent t-shirt or long sleeved shirt. Getting bad service at Chiang Mai immigration because you have a certain rude word blazed across your t-shirt is not unheard of.
Many people like to bring bulky hiking boots especially for trekking. A decent pair of trainers are just as good for this and they can also be worn during less demanding activities. Always helps to keep weight down to a minimum.
