The following list will contain all you need to travel in warm countries with provided accommodation for a long time with just a small backpack (≈ 35 liter and 7 kg).
Fundamentals
Backpack (30 to 40 liter), Waist/Crossbody bag, Passport, Wallet, Credit/Debit card, Cash (Home currency, safety), Driver license
Hygiene products
Toothbrush, Toothpaste, Towel, Nail clipper, Nail file, Plaster
Clothing
Shoes, Thongs, Raincoat, Sweater, Pants, 2 x Shirts, 2 x Shorts, 3 x Underpants, 3 x Socks (pairs), Bathing clothes
Technology
Smartphone, USB to USB-C/µUSB/Lightning cable, AC to USB Adapter, Powerbank, Headset
Additional
Pen, Writing pad, Earplugs, Razor, Shower gel, Camera, E-Book reader, Sunglasses, Base cap, Head torch, International Driver license, Goggles, Dry bag, Lock (for lockers in hostels)
For Diving
Diver license (if not digital), logbook (if not digital), Dive computer
[Rent the Diving gear at the Diving school]
For Climbing
Climbing shoes, Harness, Belay/Rapell device (+ Carabiner if necessary)
[Find a Mate with a Rope and Protection gear]
Get over there
Cash (Local currency), SIM-Card, Sun cream, Mosquito spray