Toilets. (Europeans are not squeamish about the word, unlike Americans. If you ask for a bathroom, you're out of luck). The good news is that there are public toilets all over the place. The bad news is that you have to pay to use most of them. Usually around 1/2 Euro (65 cents), but I've paid up to one Euro in some places. So travelers should carry coins with them and use every opportunity, such as restaurants, to avoid having to pay.
Uneven surfaces and steps. You naturally expect cobbled streets to be uneven, but I found that you need to look out for steps and other things to trip on or fall that you don't expect in the U.S. Things like door sills about an inch high that you can easily trip over, no landings when you open a door, half-steps that can cause you to lose your balance and fall. So you should look down as well as up.
Hotels with elevators. Many hotels will have elevators, but at some you have to climb a flight of stairs to get to the first floor (what we call the second floor) to reach the elevator. Check this before you book if you absolutely do not want to climb any stairs.