Funny, I met an Australian traveling around Europe when I was hiking in Ireland.
Generally anything east of Germany will be extremely cheap. Avoid Ukraine and Belarus. People in Poland, Estonia, Latvia, Lithuania, Czech Rep. Slovakia, Slovenia will be very friendly and will be looking forward to drinking with you. Go in the warmer months instead of the colder - snow not only means wet and cold, but also transportation problems in some regions, which always suck. Pack light and usually things you don't mind getting stolen. Have copies of your passport, documents in a dropbox account in an encrypted archive - they wont get you past border guards, but it'll be easier to get new documents at your embassy if you have those copies. Check out couchsurfing here - my buddy has had lots of luck with it in Poland and Germany, but not much luck in Sweden, but you as an exotic foreigner might have it easier and it'll save you a good deal of money. When you have a plan, try to memorize some easy convos in the languages in the countries you will visit - things like thank you, hello, etc. Many young people will know english quite well, but old people (people who usually sell tickets etc) won't know a word. Have paper and pen on you at all times.
As far as the fun part goes, I can't really say where you should go - it doesn't really matter to be honest. As long as you pick a few locations, you will have a blast. Eat the local food, make friends with the locals, check out local attractions, go to clubs. This is where couchsurfing really shines as you'll be able to see how people live somewhere else plus these people will point you to the best places to get food, have fun and will often take you out for fun (not pay for you, just take you clubbing for example).
I haven't done backpacking to be exact, but I've lived here and there in Europe and visit friends there every so often and use the chance to travel a bit. If you have questions about Poland, ask away as I'm pretty good on that area.
