Your packing list should be adapted to your traveling style :
São Tomé is a tropical island located on the Equator line. Basically it has two seasons : the dry season and the raining season. There are mainly two different weather type : in the north it is relatively dry with savannah and baobabs (also know as Adansonia trees) and in the south it is much more rainy, with clear blue sky once the rain is over ; so you may consider São Tomé to be sunny and rainy at the same time, therefore you shall bring :
For the sun :
Sunscreen, sun glasses, hat, swimming suite and beach towel (high end hotels may lend it).
For the rain :
Light rain coat, umbrella (high end hotels may lend it).
In general, you should not forget to bring your :
-camera, gopro, battery charger, snorkeling equipment., and electric plug adapter
For hike activities, you should bring :
-sport or hiking shoes, light fabric pants and long sleeves shirt
-changing clothes and socks
For boat activities, you should bring :
-small snacks and water, light rain coat (in case of wind and waves)
-changing tee shirt, snorkeling equipment
-and be wearing shorts and flip flops, since you may have to enter the water to embark or disembark from the boat (especially true for beach landings).
If you want to help local people and make small donations, then you can bring :
-Anything that is pedagogical, such as second hand games, coloring book, or any material which does not require language understanding (except of in Portuguese)
-Clothes (no winter clothes !!) and shoes (rain boots are welcome).
-Surf equipment and rash vests for Santomean young surfers.