Well, despite the EURO falling a bit lately, it is still stronger than your US Dollar. Also keep in mind that certain locales, especially in France and Italy will be extra expensive.
I would first look into the viability of purchasing a "multiple destination" international ticket. For example, something that would allow you to fly into Athens and depart from Amsterdam or something.
I would suggest something along the following:
WEEK 1 ($$):
Arrive Athens, see Parthenon.
Plane or boat to Santorini. Stay in Oia for about 2 nights/2 full days.
Depart Santorini by boat, arrive Mykonos. Stay for 2 nights/2 full days.
Boat to Athens.
Fly to Rome or bus to Patras and boat to Ancona, Italy and then train to Rome.
WEEK 2 ($$$$):
Rome: 2-3 Nights
Florence: 2 Nights
Depart Florence EARLY AM: Stop in Pisa, see the tower, get back on the train, and head north to the Cinque Terre area.
Cinque Terre: 2-3 Nights. Time to hike the five fjords in a day.
WEEK 3+ ($$$$$):
Depart Italy via Genoa, enter France.
If you can afford it, go to Nice, Cannes, and Monaco. I would say at least a good day at each destination would be nice. This is heart of the French Riviera.
Head north to Paris: Spend at least 3-4 nights there / a good 4 days minimum.
WEEK 4 ($$$):
Fly from Paris to Amsterdam: Spend a good 3 nights there.
Depart from Amsterdam to the States.
You should make the decision of whether you want to fly from destination to destination and save time (sometimes also the cheapest option as well) or opt for trains or buses. This is just a matter of research beforehand to determine how cheap the airfares are vs. a rail pass.
The above is very much a whirlwind tour.
An alternative would be just to park yourself in some exotic locale like a Greek island and just blend in and forget about the world.
Just depends on what you want/need out of this trip.