Маршрут водного транспорта Manado - Nain/Siladen/Bunaken на карте Манадо

One day trip from Manado (Dermaga Kawasan Megamas) to three islands: Nain atoll white sands, Siladen and Bunaken (Liang beach). Tour normally only goes in weekend and holidays and requires registration/prepayment. There are plenty of organisers - google for "trip 3 pulau manado" to find (some of) them.

Pro: difficult to get by yourself to these places alone for this price.
Con: group travel (mostly indonesians, little english spoken), in Bunaken you do not end up on the best snorkeling spot, and usually not much time for snorkeling altogether.

PS. A big drawback taking this trip is that you will be slapped with the Bunaken NP entrance fee when arriving on Liang beach - only 5000 Rp for locals but a hefty 150000 Rp for foreigners. The chance you'll be charged the entrance fee when coming with the public boats (see other routes to the islands) is nearly zero.