Take pictures of: bags checked-in, in case they get lost; city maps, especially public transport maps; booking confirmations; passport. A further backup in to email such pics to own email account for access in emergencies.
Bring some older clothes, like socks or a towel, that can be used and then dumped to free-up bag space if need be.
Squeeze some air out of plastic bottles carrying liquids as they are less likely to leak, especially in low pressure holds.
Some museums have free access days so look this up in advance; for example, the Louvre is free the first Sunday of every month.
Fellas, if traveling with hand luggage only bring one razor handle and then buy a set of heads and a can of shaving foam between ye. Girls, no need to bring any facial scrubs as brown sugar mixed with cleanser does the same job.
Taxi drivers will often pretend not to have change as a way to get a big tip, so try to have coins and small notes before getting a cab.
Cárta Eorpach Árachas Sláinte – European Health Insurance Card