Absolutely, extras like soft and alcoholic beverages, souvenirs snacks and tips should be settled with your cash.
The most important thing I must remind you is to ensure that your USD bills must be printed not before 2004. It is unfortunate that earlier than 2004 printed bills won’t be accepted.
You can use your credit card (visa and master card only) to make payments in some places but a 4%-5% fee will be charged while paying by credit card (most small hotels, shops and restaurants accept cash only but the big ones accept credit cards). You can also use your credit cards to get money from the ATMs (located in all the big towns), but they will only give you local currency.