I often find I need a face cloth/wash cloth, but often forget one....last time I just used it to clean my shoes rather than my face!
Shampoo, shower gel
Suncream
Sunhat
Sunglasses
Bottled water and snacks?
Paper, pens, stamps if you want to write home and for writing down contact info of any new people you meet
Shoes for walking, shoes for getting wet, some other shoes
Swimming stuff
Socks.....I always forget socks because it's summer!! But if you need to walk more than a short distance, you might want some
Pain killers and any medication you're on, and allergy relief tablets
Camera (inc. case, batteries, memory card and charger)
Something to wipe your glasses with if you wear them (I don't but have friends who do and they have been stuck when they needed to clean their glasses)
A book/something to occupy you in quiet moments
Phone charger?