Some credit cards charge a foreign transaction fee, which can run around 3 percent. This can add up quickly, especially when you factor in hotel costs, restaurants and other expenses you might be paying for in a different currency.

While you may expect to pay a foreign transaction fee for a purchase made in euros, be aware of buying items in countries that accept U.S. currency as payment. "In some cases, card issuers are assessing the fee for transactions conducted on foreign soil or with foreign entities, even if the transaction takes place in U.S. dollars," says Greg McBride, CFA, senior financial analyst at Bankrate.com.

So if your next trip takes you to the Caribbean, you could still be charged foreign transaction fees during your stay there, even if your purchases are made in dollars. To avoid these charges in places that accept dollars, consider taking a couple of extra trips to the ATM before leaving on vacation. Then pay with U.S. dollars for as many of your transactions as possible to save on credit card transaction fees, advises McBride.