Vueling best airline out of barcelona but does most cities and easyjet for carry on only luggage...do not use ryan air...its a terrible airline!!!
I found great accom in london on airbnb.com
Rome there are some great little hotels that r reasonable around the central train station just walk and ask
Amsterdam go into the red light area and you will find cheap hostels
Paris book online expedia etc but can find some good stuff...just look out for where they r...you want near the e tower or lourve!!! about 6 years ago