Samal is very near Davao City. There are lots of options for ferries going to the island. There are ROROs (Roll on, Roll off ferries) if you have your own vehicle. There are buses from Sta. Ana pier with routes to Babak and Penaplata, two of Samal's most progressive districts.
The fastest way is via the commuter ferries in Km 11 in Sasa wharf (fare is 13 pesos or .25 USD). Its a bit hard to find the ferry station if you are a first timer (its hidden behind a wet market) but taxi drivers will take you directly near the station (taxi drivers in Davao are honest, thanks to the much loved and feared city mayor)
The ferries leave every 15 minutes and the ride only takes 10 minutes. You'll be in Babak pier before you can even doze off.
As soon as you arrive in Babak pier, habal-habal (motorcycles on steriods) and multi-cab drivers will descend on you before you can even get your bearings. If you are traveling with a group or if you are the type who brings around a lot of luggage, then the multi-cab is for you.
If you are the type to likes to ride fast and hard then better get one of those habal-habals. The inner roads of Samal are terrible but the habal-habal's souped up shocks will make the ride enjoyable. The drivers liked to really rip the roads, so hold on to something, or just close your eyes.
A day's tour of the island's beaches and other sights will set you back about 600-1000 pesos, depending on where you want to go and your haggling skills. Its best if you have somebody with you who can speak the local dialect. For example I only paid 200 pesos for a trip to Canibad cove, while others paid 1000 pesos (as mentioned in their blogs).
viewpoint in Brgy Licup
There are many options once you are in Samal. There are caves to explore: the Monfort Bat Sanctuary is just near Babak pier, and several waterfalls (Hagimit Falls is one of the more popular, drop by Brgy Licup's viewpoint on the way to Hagimit).
There are zip lines if you are the daring type. White sand beaches abound, head to Kaputian (which literally means "whiteness" in local dialect) for great but affordable beaches. You can choose from nipa hut accommodations to posh resorts depending on your budget (see http://samalbeaches.com/).
Will feature photos on Monfort Bat Caves and Hagimit Falls soon.