Replying to Travelling Europe

My sister and I will be in Europe for most of August. I've never been there before. We're flying in and out of London. Trying to keep it affordable. Train passes are expensive! Any tips on how to save money? Our plan right now is to get a cheap flight from London to Spain, then go Spain-France-Switzerland-Italy-Greece. And another cheap flight Greece-London. We want to stay on the coast as much as possible. Amsterdam would be sweet but it's kind of out of the way. Has anyone taken ferries in Spain or Greece? Im having a hard time finding information about them. Also, since it's summer holidays, would it be better to book all our hostels in advance? Any help/tips is much appreciated!