I think one of the more difficult customs for visitors in my country are eating times. On weekends breakfast time is from 9:00 to 11:00 am, but during the week depends on what time you start work. Lunch time is from 14:00 to 15:30 pm and dinner time is from 21:00-23:00. This is difficult for us in return to adapt when we visit other countries. I have visited many, but I haven´t found another country with such late times for eating.
Another custom for us are the "tapeo". When we meet friends, we usually go to a bar and drink some beers together. Here our drink comes with something to eat completely free, like fried potatoes, bacon, olives or something similar. It´s difficult find this kind of situation abroad.