I posted this in tripadvisor and I thought of posting here as well.
Hello people from Europe, I have been saving for almost a year and a half for a trip to Europe and hopefully I will make it happen this summer! I would like you guys give me some ideas about the countries/places to visit and must do things etc to plan a 20 days itinerary. Helpful tips to travel inexpensively would be much appreciated! =]
Forget about Tripadvisor & other websites...
Just watch the movie named "Eurotrip (2004)" ... it's a big answer to all of your questions! XD
A good adress for train schedules all over the world
Buses may be cheaper
So the places that are cheap tend to be Portugal, Greece, Hungary, Roumania, Bulgaria, Poland
The times that are cheap are outside July and August
Eating tends to cheap if you either eat what everyone else eats or.... ethnic food (ie not of the local ethenicity) or.... vegetarian food. Don't drink international brands of wine/beer only local and don't buy water, ask for tap water or drink tap water (probably healthier for you).
Don't travel much as that can add up but look at seat61.com to understand all the train concepts, add rome2rio to get good planning overviews and consider student rail cards.
It really, really depends on what you want to see and what you enjoy doing. In addition to the great suggestions so far, you'll get perhaps some additional ones if you tell us a bit about the places you've visited (elsewhere), which were best in your opinion and what you've liked/disliked during your travels so far.
That said, Interrail still seems to be going strong and trains/buses will certainly get you to most places, even the smaller villages you might otherwise not think of visiting. And if you're on a budget, I'm afraid you'll have to skip Switzerland. ;)