Here are thoughts on the subject as they came to me:
Know about a country's currency. Certain countries like Zimbabwe and El Salvador accept only US dollars as they do not have a monetary system. If that's the case, bring crisp new bills in small denominations. Your bank will help you before your trip. Plan ahead.
**Remind me to tell you the story of my backpack full of money envelopes getting a total search by a diligent employee at TSA in Rochester on my way to Africa. Talk about being vulnerable in a public place after that.
Use an international money converter app on your phone.
You may, or may not, take a small amount of Euros with you - cab fare - for the start of traveling in the EU. It's up to you. Again, plan ahead with your bank. Most bank branches in the US don't keep Euros on hand.
ATM's are available everywhere. Use one only in a lighted place and not at night. Often your hotel front desk staff will suggest one in the vicinity that is user-friendly for travelers.
**Sometime I will write about my escapade in Madrid, Spain at an ATM. I inserted my card and neither did I get cash, but my card was in the machine, too. Fortunately I was with an American couple and they stayed with the machine while I went inside to talk with bank personnel, who could barely speak English.
Avoid hotel and money change locations if at all possible as their rates are higher.
Don't forget to call your bank to put a travel alert on your debit card. Include airport layovers, too.
**When I was an inexperienced travler, I didn't do that and was I embarassed at the outside table in Venice when the waiter returned my card. Luckily the bank contacted me by email and it got straighten out. Whew! Lesson learned.
Keep your money spread into several places such as a money belt on you, wallet etc. Use a small change purse for currency that you can easily access and won't draw attention. The hotel safe works for larger amounts.
Tipping is not big outside the US. You still need small amounts for bottled water and basic supplies, however, and for housekeepers.
Scandanavian countries prefer credit card transactions. Travel is made easy that way when you can charge a pack of gum. Sweden will be cashless soon.
In general, use a credit card that has no international transaction fees, or at the least, very low ones. Keep the slips to double check your bills when you get home.
Unless you are buying expensive items, you don't have to declare under $800. at US Customs.
A final fun tip: Save the last bit of of foriegn currency to spend as mad money at the airport while waiting for your plane. I've returned with some neat items I never knew I "needed."