Acceptance per country
In Nordic countries (Finland, Sweden, Norway, Iceland) as well as in Switzerland and Liechtenstein, you will need no cash at all. In those countries all businesses will accept cards. In the Nordics there are also many cashless businesses, so a card may be the only way to pay.
In Italy, Spain, Poland, France, the UK most businesses accept cards. In Italy in particular it is mandatory by law for all stores and restaurants to accept cards. When traveling in those countries you don’t really need to have any cash on you, except for a couple of coins for things like public toilets, luggage lockers, laundry machines, vending machines and the like. Those are still often coin operated and may not take cards.
In Germany, Austria, Czechia, Slovakia you are more likely to encounter small businesses, like small restaurants outside of big cities, that only take cash. In those countries we recommend to keep a small amount of cash with you.
In Portugal and most Balkan countries card acceptance is not that good and you definitely need to have some cash on you.
Types of cards accepted
Visa and MasterCard (including all their flavours like VPay and Maestro) are universally accepted. You don’t need a credit card, a debit card will do.
American Express might be accepted in some places (mostly higher end stores and international hotel chains etc). Discover, Diners Club, JCB or UnionPay cards are very unlikely to be accepted. If any of these are your only card, you should acquire a Visa or MasterCard for your trip.
Most payment terminals accept Contactless “tap” payments, which means you can pay with Apple or Android phone or watch.
How to get cash
The best way to get a local currency is to use an ATM. That is usually cheaper and safer than using money exchange services.
Note that only the local currency will be accepted. In some countries you may be able to use the currency of a neighbouring country, for example in Switzerland many businesses accept Euro, but they’ll accept it at bad exchange rate. So if you wish to use cash, make sure to have the local currency. US dollars will not be accepted anywhere.
How to avoid getting scammed
Before using an ATM or paying with a card abroad, do watch that video: https://www.youtube.com/watch?v=v1Hh8kY_hOY. TL;DR is: the ATMs and card terminals will often present you with a choice to pay in a local currency or convert it to your currency. It is always a better deal to decline the conversion, that is, always pay in the local currency and not in the currency of your home country.
If you want to exchange cash, always use exchange services. If you are approached by a person on the street offering you to exchange money, never agree.
What card to choose
Any Visa or MasterCard will work. However pay attention to fees for foreign transactions imposed by your bank and to any markup on exchange rates. In some cases, you may need to inform your bank that you are travelling for your card to work.
If your bank doesn’t offer you a good deal for transactions abroad, the best travel card options avaliable to you will depend on where you are from and not where you are travelling to. Therefore, you are best to check or seek advice from a subreddit related to finance in your home country.
Some major international options are services like Revolut or Wise. Before creating the account, ask for a referral code at https://www.reddit.com/r/referralcodes/ — that way you may save a couple Euros.