Goomba, which airline will you fly transatlantic ? Maybe they can offer a skypass in Europe ? This is just idea
Another idea is to book a flight+hostel package with a travel agency, it could be nice in september as this is not the peak season...
If you want to travel with LCC, try to avoid weekend flights and also take care about the different airports, for example Beauvais (Ryanair) is 80kms from Paris.
For Amsterdam-Paris, I think the best option is to take the high speed train, the journey is not very long, you are right in the city and you can have a stopover in Brussels if you want...
Have a nice trip.
: don't know if this can help you but here's also another website not mentionned above : www.vbird.com