With London/Paris travel time on Eurostar down to 2 hour and 20 minutes, a daytrip is now possible! Take the first train early in the morning, see a few sites very selectively, then go back on the last train. This will make a very long day. Remember to factor in the 1 hour time zone change.
For full appreciation, each city deserves several days or even a week or more.
If your time is limited, Eurostar makes a day trip a possibility to consider.
-Paul