I know there are tours offered from Paris to Normandy as a day trip. Is it worth it?
Taking a trip to Normandy is absolutely worth it. Depending on what you are going there for it might be a bit far for just a day trip. I was there 2 years ago primarily to see the D-day beaches and Mont St. Michel and 4 days was not enough time. The next time around we will be renting a car in Normandy.
"Is it worth it?" That depends on what you want to see. "Normandy" is a big place, and on a day trip tour, you can only see a bit (and at high speed, going from place to place). If that bit entices you, and you don't mind the expense of the bus tours, go for it. If you want to see more, you will need to plan a separate trip of more than 1 day's duration.