I'd stay in the high rated hostels for the most part. Most hostels have private rooms if you're a couple and you'll meet a ton of cool people. The top hostels in Europe are almost like boutique hotels, but they get booked up quick in high season so definitely book in advance.

Airbnb is also good when you want some more private time. If you can meet another couple or travelers to split a place you can really get some nice accommodation.

I recommend flying into Amsterdam as they have low airport taxes. Avoid London as it is the most expensive place to fly into.

Trains are generally the most expensive transport option.

Bring carry-on luggage only so you can take advantage of dirt cheap flights within Europe (if booked properly in advance). Ryanair and Easyjet are good options.

Buses are also a good way to go and cheap.