Cash

Cash is useful for paying tips (restaurants, cafes, tour guides), for small expenses and also for paying taxis who generally don't like to be paid by credit card. Also some local buses do not accept payment by credit cards.
So one day it might be only €5 for a tip in a restaurant, and another over €50 with a taxi ride plus a tip for a guide, plus some small expenses.

Anyway you can withdraw cash from an ATM if needed.
Note that each withdrawal operation is charged with a €2 to €3 fee (depending on the bank), so make for example a single withdrawal of €100 instead of €50 one day and €50 again 3 days later

We never carried more than EUR 100-200. Same as here, most businesses take cards, and ATM's are ubiquitous.

I did that tour starting the end of the first week in September, and then went out to Kea for a week long cooking and food thing. I was surprised by how little cash I used. Once, I tried to use a 20 Euro bill to pay for a bottle of water because I didn't have any change. The grocery store clerk insisted that I use a card to pay for it (30 cents). I was glad that my card didn't have foreign transaction fees. Due to the pandemic, virtually all businesses now take cards. That being said, I had booked transportation to and from the airport, as well as transport to the ferry port online in advance. That being said, if you do a lot of shopping for souvenirs in small shops, you may need more cash than I did.

Ricka

If you use an ATM to get some currency you will be prompted to choose whether or not you want to opt for the currency conversion at the time of your withdrawl. DO NOT accept this conversion, as you will be charged for it and the exchange rate may vary but not in your favor. Let your bank calculate the conversion rate for you.

To complement Lee's warning, check out this thread about ATM withdrawals in Greece:

Thank you so much! I think I know what to do now.