Here is one plan

I am pro-train, but have my limits. The rail routings from Prague to Paris make for a very long train ride and likely exhaustion on arrival. On the other hand, I flew a Czech discounter called Smartwings from Prague to Paris in about 90 minutes, the best decision of that trip.

Geoffrey,

A more efficient method would be to use open-jaw flights. To begin with, fly to London, take the EuroStar to Paris, then budget airline (SkyEurope from Orly) to Prague. Finally, fly home from Prague. If you can't find convenient flights from Prague to Portland/PDX, you could try flights from cities that are a bit closer to Prague such as Berlin, Vienna or Munich.

This avoids the time and expense of returning to London and will provide at least one more day of sightseeing.

Cheers!

Geoff, Your trip can be whatever you want it to be. Personally, I'm a do it all, see it all person too. Our first trip to Europe was in 2000; it was five weeks, the last two in Italy only. Our group was my husband, our 15 year old daughter and myself. I planned everything using Rick's books and the internet. You can trust Rick's advice. Everyone thought our trip would be too intense, but I found that to be part of the adventure; I knew if I only went a few places in all that time, I would have wished I had seen and experienced more. For us, the trip was perfect. We started in Ireland for five days with a rental car, Dublin to Galway, to Dingle, to Blarney and all the sights between. Ferry to Wales, train to London (3 nights), train to Paris (3 nights)then Chamonix, Berne, Tirol (neuchwanstein castle), Saltzberg, Prague, Krakow, Vienna, then Italy beginning with Venice, rental car for two weeks, ending in Rome. Yes, you will go back, and we have, seven times! But I don't know if I'll ever have such a block of time off; and our trip gave us a feel for all these places. If I had three weeks, I'd divide it amongst six destinations, at least. I think three days is more than enough to get introduced to a city and see it's main sights, but, like Rome and Paris, you'll never see or experience all they have to offer! And there are wonderful places to go to just for a day. For example, we left Paris, early AM, arrived in Chamonix in the afternoon. Walked around town, did laundry, delightful dinner. Off to bed, up early to share breakfast with the hikers and ice climbers, caught the first gondola of the day up Mt Blanc, back for lunch,to the train by 3pm. A fantastic 24 hrs! We count our travel time on trains as part of the fun, too! Your can enjoy the fantastic scenery, nap, play cards, read, picnic, etc. as well as have great converstions with locals and other travelers alike. Europe changes you. Experience as much of it as you can, savor the memories!

6 days sounds a little long for Prague, unless you really can't get enough of souvenir stands. Perhaps if you did one or two daytrips, though, it would be appropriate.

I think it would be dead simple to fill 6 days in London and Paris, ( you could do daytrips from each city) but Prague I hear is kind of a 3 or 4 days stop.

I agree staying three or four days in Prague. That's plenty of time to see things. The people in Prague are very unfriendly!! But it's a beautiful city.