Samal Island lies directly across Davao Gulf from the city, reachable in under 20 minutes by small motor ferry from the Santa Ana wharf. It is the easiest escape from the city and has good beaches, clear water, and a fraction of the development you would find in more visited Philippine beach destinations.
The best beaches are on the western and northern shores. Coral reefs close to shore make for decent snorkelling without a boat. The island also has the Monfort Bat Cave, which needs to be visited at dusk for the full effect of the evening bat exodus. Day-trippers can easily combine a beach morning with the bat cave in the late afternoon before catching the last ferry back.