To me the Normandie beaches are not really a day trip from Paris. I know people do it but my word, it takes a full day just to see some of the sights with a guide leaving from Bayeux!
Giverny is very easy to do on your own. And I say this as a person who has never lived where there has been public transportation, lol! I have pretty low travel skills but they are better in Europe! I've done the train out to Giverny and it's pretty easy.
If you enjoy medieval cathedrals and stained glass, Chartres Cathedral is a wonderful day trip as well.
Last time I also went to Chantilly for a day trip and enjoyed that as well.
You could theoretically do a day trip to Brussels on the fast train. It's a little over 2 hours. I'd not really do that but again, people do.
If you decide to use a company for a day trip, know that Viator is a 3rd party booker. With some research you can usually find out which company runs the tours and book directly with them saving you or them the money Viator takes for their cut.
I've got both of the previously-mentioned Annabelle Simms' books - One hour from Paris and her newest one, Half an hour from Paris. Both have interesting day explorations.
What kinds of things are you interested in?