Cash is king: most of South America, Southeast Asia and Africa
In these areas cash remains king and you may find that even larger hotels do not accept credit and debit cards. If you are planning to cross any borders you should also expect to need cash for visas and should not assume that there will be an ATM nearby.
Many of these countries have currencies that are either unstable or unpopular, meaning that the US dollar is often accepted widely.
In Vietnam for example, you will even find hotel rates quoted in dollars. Check before you travel and if the US dollar is widely accepted take both dollars and the local currency.
Make sure also that all bills are clean and undamaged, as well as in small denominations such as $10 and $20. A hotel in these destinations may only set you back $20 and a meal $2, so you won’t have much use for $100 bills.
As you may need to pay for everything in cash, even more forward planning is required in these areas. Try to work out a budget for your entire trip, assuming that every cost will need to be paid for in cash, and order the currency you need in advance so that you will have it when you arrive and won’t need to hunt down an ATM.