Hi Carol. I can give you a fellow American's view although the Europeans can give you the best info.
We just went to Paris at the end of Feb-beginning of March. We stayed at the Citadines Aparthotel on the Seine. The location was awsome!! My DH was in meetings on some days and I was able to walk a bunch of places and find my way back easily because the hotel is located about 2 blocks from Notre Dame one way and 2 blocks from the Louvre (sp?) the other. For seeing Paris city sites this location couldn't be beat. If you want to stay at DLP we spent 3 days at the DL hotel which is right at the entrance to the park. Again, location, location, location! THere is a train stop (I think it's called the RER) right between the entrances to downtown Disney and Disney Studios. We never used it from there but I do know that it was easy to use when we used it to get from the airport to Paris and around town. One tip: have 1 and 2 euro change so that you can buy your ticket from a machine. The wait is much shorter.
We tried to take the Eurostar over to London for the day for 120 euros round trip but because we didn't buy our tix in advance the price was going to be 450. OUCH! Needless to say, we didn't go so I can't help you with that one.
My DH just got back from Barcelona. I will try to remember to check with him to see if he has any tips for you.
Enjoy!!