Cash Is King

My first piece of advice for traveling is quite simple and the reason is simple. Take cash. Why? Because international ATM fees can be ridiculous. 
The reality is that many stores and vendors do not accept credit or debit cards so cash is a necessity. Also, I prefer to use cash at smaller vendors that do accept cards because we have had our card numbers stolen several times in the USA (not internationally, yet) and while my bank is very good about quickly returning the money, it’s much easier to just avoid the problem. 
My bank offers foreign money if I go to the bank and request it far enough in advance. I did this for our trip to Canada because the US dollar is worth a little more, and some places post separate US and Canadian prices but not all. Many places in touristy towns will happily accept the US dollar as if it were a Canadian dollar so you’d be essentially paying more for items.
If you choose to use the US dollar in cash to purchase in another country, take the time to make sure that prices are in US dollars. In Mexico, for instance, if the dollar sign has a double vertical line, that is NOT a US price. If you are in a place that doesn’t post the US dollar price, be sure that you know the exchange rate. 
If you need to use an ATM, there may be an advertised fee that you are willing to pay, for example, in one country, the notice on the screen asked if we were willing to pay a three dollar fee. When we returned home, our bank charged us a foreign ATM fee and a miscellaneous fee for a total of an additional ten dollars.
Many worry about getting robbed. Having travelled to over 10 countries, I have never been robbed. That being said, I take precautions. When on our way to the port or airport, I make sure that my carry on bag has a small pouch with money I’ll need immediately. I try to have about ten ones, three or four fives and a couple tens. This stack may be a little bulky but the money goes fast. Florida has tolls, we have to tip baggage handlers, we may need a snack or drinks for medication. I also keep the rest of my money in my carry on in a different place. Again, I have about twenty ones, a bunch of fives and tens and twenties depending on exactly how much cash I’m carrying. Because I ultimately want to avoid ATM fees, I take way more cash than I intend to spend. I’d rather come back and deposit my surplus than have to withdraw out of the country. If you do have to withdraw money at a foreign ATM, take out as much as possible to avoid having to withdraw internationally multiple times. If the ATM gives you large bills, try to find a safe place to change this to smaller bills immediately; this may be difficult.
The most important reason I keep so many small bills is s that I don’t appear to have a lot of money; that last thing you want as a tourist is to be seen with a roll of hundreds. I fold my money in half so that they’re almost squares and twist it into a tight tube, then place it into a prescription pill bottle or two. I put the pill bottles with money with the rest of my medication. With the label covering most of the bottle, most people won’t notice it’s money on a quick glance. If your accommodation (ie hotel room) has a safe to keep the money when you get there, even better. Whenever I leave my room, I only take the amount of money I have budgeted or expect to spend. 
When I do use my bankcard, my bank does the conversion rates for me and sometimes I’m surprised that the purchase was a little less than I expected. Many may read this and think it’s just easier and safer to just not carry cash. After all, you could get robbed and it takes a lot of mental energy to worry about the conversion rates. When I travelled out of the country a few years ago, I thought I’d beat the system and take a visa gift card because it wasn’t tied to my bank account.
Keep in mind that not every shop takes US credit cards or have a card reader machine at all. Also, many gift cards do not work internationally. And, is that gift card insured if the card number is stolen? Most are not. Also, when you spend cash, you are have the tangible representation of what you are spending as opposed to just swiping a card. Cash is king.