Socks, a generous supply of underwear or swimming gear, belts, extra pair of flip flops, hair bands, sun glasses, wash clothes – these are the small items we usually forget but are extremely important while traveling. These are things that you can find anywhere in the world but perhaps not when you need them the most. It's better to carry them with you because you might end up needing them at a moment's notice.
Tampons or pads are often forgotten about, but are obviously crucial if you menstruate (and may be tricky to find once abroad, depending on where you are going). Your period might always be on time, but it is also a good idea to carry a supply with you jut in case.