cheapest most of the time is by bus however not a desirable mode of travel for many. Ryanair is good (once got a return flight London-Marrakech for 30pounds with them!) however sometimes the airports they use are out of the way, I know for Frankfurt it is. Here is my method for finding the cheapest flights which served me well travelling europe and africa over 3.5yrs.

Go to skyscanner.net or jetabroad.com (these are the ones i used that i found the best but there are many) Check the flights you want and it will list all carriers from cheapest to most expensive. Afterwards when you find the cheapest flight, lets say easyjet, you then go to the easyjet site and see how much they charge. I’ve found flights that were say 30euro on skyscanner and then on the carriers site are much cheaper, like 15euro, and equally its been the other way around.

skip Barca, people are rude @rseholes, my least fave place in europe (however sagrada de familia is amazing). Madrid is cool though, its spain, barca is not spain, Catalonians suck. Otherwise a good itinery. Germany is my fave country, many good times had there. Try and add in Heidelberg if you get the chance…its awesome

EDIT: Also i’d highly recommend Prague (cheap trains from germany) Prague is a really fun and beautiful city. Bern and lauterbrunnen in switzerland which is on route to marsailles-barca from germany

Eurolines is the long distance bus operator. They aren’t luxury coaches, but they get you places for cheap.

Check in at the train station. I have seen cases when fares are in line with bus tickets. Ask if fares are different a day before or after you want to travel if you’re flexible. I’ve caught 50% discounts by changing my plans by a day or two. But on average trains aren’t the cheapest route.

I didn’t like the eurail pass. It ended up being more expensive than it would have been for us to travel by local transportation. And it really locks you into traveling in a certain way as dictated by the terms of the ticket – it’s just inflexible.

Ryanair and other airlines as mentioned in another great answer offer very cheap flights around europe. Air is probably the best option for longer legs of your trip. Just check the baggage limits. They are often very restrictive. But even with extra baggage charges it’s likely you’ll find Air the best alternative in many cases (such as your jump from Germany to Spain).

My favorite mode of transportation is just to take local buses and trains along a route made up of reasonably close destinations. It’s more fun than missing out on all the scenery at 30,000 feet. I do suggest one route from germany to spain by local train going down through Switzerland (Zurich), through the alps (what a trip on train!! just looking out the window is spectacular), stopping in Northern Italy (Milan), and heading along the French Riviera (Nice, Monaco, etc) on to Spain. I did that trip in reverse order over about 2 weeks, it was great. But you’d need a decent amount of time to make that route.

Enjoy your trip!