@TheM55 that's really good advice, but not quite right on passport rules.
Also travel insurance is a must, otherwise you'll be in a huge mess if you have an accident where you can't fly home as normal.
For money, I would pay by card just about all the time (look on Moneysavingexpert.com for advice about which cards are best and which to avoid, the latter ones that charge a fee per use, so massively expensive if used to pay for every coffee, bus ticket etc).
But get a small amount of cash from a bank cash machine when in the city, £20 or so as small independent businesses often prefer cash for small purchases. I've never been to Hungary but found that in Spain, unless you withdraw from a Santander bank, using a Santander card, which is what I do if I can, its virtually impossible to avoid paying to withdraw cash.
I didn't pay to withdraw cash in a Polish city, but did find that public toilets were changed for, mostly cash, but there was one in a hotel that was open to the public and could pay by card.
But also ask around friends and family in case they have some going spare, I've always got spare euros lying around and currently have about £14 of Zloty with no immediate plans to return to Poland although it is likely I'll go at leader a couple of times for short trips in the next few years.