A camera.
Comfortable shoes as you’re probably going to be doing a lot of walking.
If you’re visiting during the summer, a sunhat is very useful, and loose, comfortable clothes will help you cope with sightseeing in the heat.
Some small change in euros for your arrival, handy to use for taxis, tips or an airport trolley.
Plug adaptor – Voltage is 220 and the plugs are two-pin round prongs, not flat.