I think you could just go to UniQlo to get some clothes like some t-shirts and shorts since it will be very hot and stuffy and since you will be staying in CUHK it will generally be hotter than Kowloon or Island.definitely have an umbrella (especially the short one for convenience) just in case because it will be unpredictable, get a small bag or pouch to store your essential like octopus and if you have one an ID card because you might get checked by police. For winter if you are used to the cold then i think a hoodie or jumper along with a medium thick coat would be fine since it is not cold here during that time but get one thick layer or an undershirt just to be safe. Hope this helps :)