A universal electric plug adaptor and a universal sink plug.
A sheet or two (if staying in youth hostels).
A small flashlight.
Earplugs (for street noise in hotel rooms).
High-factor sunscreen.
A pocket alarm clock (for those early morning departures).
An inflatable neck-rest, to help you sleep on long journeys.
A multi-purpose penknife.
A needle and some thread.
Plastic bags (to sort your baggage, make it easier to pack and unpack, and keep out damp and dust).
Items for a basic first-aid kit are listed under "Health".
