Imodium - it’s all very well saying there are pharmacies and supermarkets abroad, but you have to be able to get there! Difficult when you can scarcely leave the bathroom!
Antihistamines - we both get mild hay fever, and they are supposed to relieve the itching of mosquito bites, taken regularly.
A microfibre towel - I often use one as a picnic blanket on a beach.
A waterproof pouch for my phone, room key, etc.
Spare glasses - while I have prescription varifocal sunglasses, it’s no fun trying to read menus with them, in a darkish restaurant in the evening!
Nail scissors (in the hold luggage) - for broken nails, removing clothes tags, etc.
Water bottle - I feel guilty about buying loads of small plastic bottles on Greek islands, where waste disposal is a problem.