Since you only have a short time frame to work with, the important thing is to spend as much time sightseeing and less time in transit between places. There are so many incredible places to see in Europe (been there 6 times), but proximity between places is the key issue here. Due to this factor, I recommend doing London and Paris only. The Eurostar speedtrain can get you between the two cities in just a couple of hours, so a whole day would not be wasted on transit time. Both cities are magnificent with tons of things to do. Have fun!