Where can one change or withdraw money?
Not on Bunaken up till now anyway. We use to draw our money in Manado at the cashpoint of the Bank of Asia (BCA) by our VISA card in Indonesian currency before our transfer to the island. This is free of charge. Have al look at the internet page of your bank and ask about the conditions or partner banks worldwide. You can usually exchange cash (U.S. $ or euro) very favourably at money changers into IR.
