(With apologies, sferik, just thought it might be a better format.)
None, I prefer to live cash-only. And yes, it's possible.
I think most countries legislate that you have to accept cash for debts. Are there any circumstances where its more difficult to use cash than credit?
Buying drinks/snacks on planes in the U.S. It's not universal, but probably 50% of the domestic flights I take specify that they only accept debit/credit cards for payment. On a recent flight that did accept cash, I overheard a passenger ask why this flight took cash and others did not, and the flight attendant said that their particular plane hadn't been fitted with some system that would allow credit card checks in flight, but it was on their maintenance schedule.
Hmm, that's kind of odd. I'd have thought international flights would be credit card only, but domestic would accept cash since its all the same country/currency.
Pretty much every European flight I've been on accepted cash (euro + local currency sometimes) and card payments.
1 debit, 1 credit. I only use debit one. I never use cash.