Whatever you need for temperatures between 15° and 30°C, including swimwear.
Rugged clothing is advisable. Formal evening attire will definitely not be required.
A flashlight – ideally a headband-mounted one – can be very handy. A Leatherman tool or Swiss army knife can be useful. Sunglasses are absolutely essential. A personal drinking bottle is also a good idea.
You should keep medication (diarrhea and cold meds, vitamins, etc.) and other personal items (sun blocker, insect repellant) within reach.
Sun protection – including a hat and sunscreen (which is often unavailable or extremely expensive in our destination countries) and SPF 30+ lip balm against chapped lips – is a must-have.