20 bucks a day is very realistic depending on the countries. In Western europe, it's very tight and you'll probably have to live really poorly to spend only 20$ a day. A hostel in Dubrovnik in the summer is 55$. Not cheap.

By far, the biggest expenses are accommodation and then transport. If you've got a place to sleep, it's really not hard to not spend much money. Evey city has loads of free activities to do. And for 10$ you can get a good meal in most places. If you don't eat out, then you're living off 10$ a day for random coffee, tips for free walking tours/museum contributions/souvenirs.

I'm not a huge fan of this guide because he recommends sleeping in a laundromat/bus station as an accomodation option to save money. I mean, sure sometimes you're on a low budget and didn't find a cheap place to sleep so you're forced to sleep wherever. But I doubt many people ever went "I want to travel for cheap, i'll just sleep in laudromats". It's not really an option, it's something you do because you have to.

Do try it out one day, hitchhiking and couchsurfing is crazy fun :) Where would you go ?