6 weeks and where to go?

that is ambitous, you are covering alot of ground and even though you have alot of time I still feel that you are leaving yourself short in some places. Are you going by plane or train between all these locations? I just worry that you might feel too rushed....
Looking at this I see that you could extend the marseille part of the trip and explore more of the Provence area. I would consider adding a few days in London and paris as well.

I would definitely add a few extra days in either Paris, London or Interlaken. If you are traveling by train or bus, you can always do day trips from these locations and take in the flavour of sites outside of the big centers. There's a number of interesting things to see that are within an hour or two. Cantebury Cathedral is 1 hr away, Oxford and Cambridge are also close to London. Versailles is close to Paris, and I'm sure there are lots of other interesting destinations you could add. I have a soft spot for the Interlaken area, and can think of a million things to do that take more than 2 days (boat trips, hikes from village to village, Ballenburg--out door museum, gondola rides up the Jungfrau or Schilthorn, steam train ride up the Rothorn). Sounds like a great trip !!

I'd stay an extra day in Venice and the rest in London. You can take some great day trips from there to Bath, the Costwolds, Stratford, Oxford, etc. I've been to London twice and stayed 3 or 4 days both times. I don't feel like I even come close to seeing everything I want to see. Also, 1 day in Venice is a little rushed. You can certainly see the main sights but even two days won't leave you a lot of time to wander and get the true feeling of the city.

Add at least a day to Paris. For one thing, you'll want to see Versailles, which is a long train trip "outside" of Paris. (And visiting Versailles alone also requires half a day.) Also, you'll want time to soak in the Parisian way of "slowing down" and enjoying people-watching at a café with a good cup of coffee. Sure, Paris is also so jam packed with things to see and do that 4 days might be rushing it, but Paris is something not to be rushed in order to be enjoyed, as I found out. Vancouver to London discount airline....maybe have a look at Zoom (www.ZoomAirlines.com)

You could add days to your London or Paris stays and takes some day trips to nearby cities. Or you could go to Ireland, it is really a beautiful place to visit.

Yes, I agree add more days to London and Paris, and definitely more days in Venice. You might find Venice a hassle to deal with since it is very overrun with tourists in the summer, but if you can stay at a hotel in the city and spend the nights there strolling around, it is magical. Rick is right about this.
I have been to London twice and barely scratched the surface of things I want to see, same for Paris, though I have been ther 4 times and have seen a lot -but just hanging there drinking coffee is the greatest experience in the world -- could do it for weeks.
Hope you have the energy for all this!

You have 2 slack days left. For this type of trip you'll want to add an extra day to have more time in places you haven't been to before. Time for shopping, cafe watching, laundry, errands, etc.

My favorites would be Paris or London, and Switzerland.

Berlin is a unique city and unlike any other of the European capitals. No place else was more effected by the cold war and the iron courtain. 18 years after the fall of the wall that brutally cut one city into two parts, one country and one people into two the city has achieved so much in rebuilding, remodeling. There is no other place where within a few yards radius you can see main settings of the Prussian kingdom, the German empire, Hitler's nazi terror regime, the totalitaritan East German "soviet" regime with their Stasi-terror grous and of course the main settings of the fall of that iron courtain. There are plenty of flights from Greece to Berlin and Berlin on to London. Check Condor airlines, Air Berlin, TUIfly.com, easyjet (but watch their baggage restrictions), LTU and others. Check skyscanner.net

At first I agreed with the earlier posters who said to add an extra day here and there. Personally, I like to travel at a slower pace. My brother and sister-in-law recently spent a week in Cologne, Germany. They had a great time, exploring neighborhoods, taking day trips and drinking the local beer. At present my son is working on an organic farm on Portugal's Algarve in exchange for free room and board. He also enjoys "hanging out" and visiting with the other tourists... Then I reflected on a European vacation that my family took in 1966 where we saw Paris, Amsterdam, Vienna, Oslo, Stockholm, the family farm in Sweden, and many other sights. I really enjoyed that. Actually, it was my family's first and last big vacation. So, on the hunch that you want to experience as much as possible and have an extraordinary energy level, I'll second Andreas' suggestion to visit Berlin. Berlin is amazing for all the reasons Andreas mentioned.......

I think you have a superb trip planned. My wife and I like to do it this way too, see lots of things and not just hone in on one like many do. But, we don't go to relax, we go to see and do.

I'd fly to Budapest from Venice and stay there 3 nights as it's a very relaxing and gorgeous city with great rests, etc. Be sure to enjoy the mineral baths-they are FAB!

You can use this site for cheap airfares-it's great...

http://www.skyscanner.net/

Thanks to everyone for their superb ideas and information. Our trip is now planned and we are ready to see the sights and have an awesome vacation!