so many choices. The suggestions in the other answers are great.
Versailles is a great day trip if you are interested in Louis XIV and the opulence of the French court. Its about an hour by train each way, and everything is in easy walking distance. Eat at a restaurant outside the chateua for better food. If you go on a Sunday you can see the fountains.
Chartres is a nice town to visit, has the big cathedral, also about an hour each way.
Rouen is an easy trip, has the cathedral that Monet painted a lot, and a market in the plaza by the cathedral on several days in the week, and of course a Joan of Arc town, where she was examined and then burned by the church.
Bayeux is a really nice town, for history the cathedral and the Bayeux tapestry. Lots of cool little shops to visit as you walk down the main street.
One that hasn't been mentioned yet:
Reims is an hour and a half away - has the cathedral where all the kings of france were crowned, beautiful church, plus lots of champagne wineries to visit. Also an easy place to walk around and enjoy. Epernay is also a nice town in the champagne region, a little closer to Paris.
There are buses that will take you u the Normandy coast to see the WWII places - the American cemetery is beautiful and moving, and Pont du Hoc is interesting, and there are several places in between. The beach is also gorgeous, wide flat beach.
Fountainebleu Chateuax is also a day trip to the southeast, visiting another amazing place and grounds. The concierege at your hotel should be able to help you with the travel arrangements, or if your french is good you can do it at the train stations.
Whatever you choose, you should have a great time.