That said, Rouens is awesome. It's maybe an hour train ride.

with its beautiful stained glass windows are about a couple hours R-T from Paris. The awesome Normandy beaches and towns to the north near the Atlantic coast, and the peaceful, majestic Loire Valley Chateau are a bit longer day trips but well worth it. It depends on your interests, time schedule and the seasons.
My top two favorites now are Normandy & Giverny, both incredible. I'm lucky enough to have seen all sites mentioned.

or have additional time elsewhere in France.

My husband and I spent 4 weeks driving around France on our honeymoon 30 years ago, and we've been back
to Paris several times since then.

I do think that visiting Giverny was one of my favorite day trips (not on our honeymoon), but we were there in the spring
and the gardens were gorgeous.

I also loved seeing the Chateaux in the Loire Valley. And going in some of the caves to see the prehistoric art in the Dordogne/Les Eyzies was mind blowing, but
probably too far for a day trip.

If you are a foodie, then take the TGV to Lyon and spend the day there.

There are numerous cheap round trip flights. I go down there for the day from Düsseldorf all the time, and it's twice the flying time. I was just down in Paris two days ago--it poured, and although I got most everything done I wanted to, I was happy to get outta there!