score:5
This depends where exactly you are and what you will pay for.
I haven't been to Serbia but in Hungary, Romania and Bulgaria you can pay in Euro in bigger cities often. But it heavily depends where and what you will buy. Paying the Hotel/Hostel/B&B will most likely work. Same for some bigger food stores and tourist attractions. Often there is also a price declaration in restaurants in Euro too. Sometimes you'll be charged with an extra fee for paying in Euro. Also people in local markets selling vegetables or fruits are happy if you pay in Euro. I made the experience that people running independent businesses are quite happy if you pay in Euro and in general people in eastern Europe even in rural areas "prefer" Euro - but don't rely on this too heavily. Paying in Euro is always a bit more expensive sometimes much more.
Note that in general you will always need some change in local currency if you want to use public transport.
Don't use currency exchange offices. Prefer banks instead. If you have to use the offices make sure to check the amount you got by the clerk.
My solution for this is to have two German online bank accounts. At DKB and ING-Diba you can sign up for an account for free. You'll get a credit card for free and can use it to get cash at every ATM in Europe or in case of DKB worldwide without paying fees. You can get the local currency from the ATM at a reasonable or good exchange rate. Traveling with two cards with not too much money on each was a always a good solution for me, especially in eastern Europe where a card can "get lost" easily (never happened to me). You can pay with your card or get cash nearly everywhere without worrying about exchange rates. But make sure to check your transactions when you are back home. Sometimes there are some transactions "accidentally" booked wrong to your disadvantage. My bank was always very obliging to compensate this.
I don't know about Spanish online bank accounts but may be this is an option for you to go.
EDIT: Note the comment by Szabolcs. He has a different point of view regarding Romania.