I loved Spain and I think most people here who have been there before would agree - don't pass it! If you're starting in A-Dam, you may want to go up and do London first, and then work your way down. You could go from London to France to Spain, ferry over to Italy, ferry over to Greece. You can get discounts on the ferry via a Railpass (view my signature to help you decide if it's worth it). There are festivals ALL the time all over, so don't worry about that! It would be hard to miss them.
If you want to stay away from tourists, you're going to want to avoid major cities (A-Dam, London
) because you'll be venturing there around the start of the peak tourist season. Aside from these cities, to avoid tourists, try hitting some cities/towns maybe an hour or so away from hot spots. Things are usually less busy and cheaper, but you still get cool stuff to see and do.