Day Trips from Paris and Rome

Orvieto from Rome is a small hill top town as a day trip
Chartres or Rouen or Provins from Paris as daytrips.
The coastal towns are a little too far for daytrips from Paris.
Someone may have a suggestion for a coastal town as a day trip from Rome,

Ostia Antica is a nice day trip from Rome. It used to be an important port city near Rome and the ruins are quite extensive. I recommend a guided tour. Almost better than Pompeii in the sense of there being more complete ruins and mosaics giving you a sense of past life. Plus trees that for obvious reasons Pompeii does not have. It is a beautiful place. I would check Walks of Italy for possible tours.
Paris- Giverny is a wonderful place - gorgeous gardens and Monet’s house where he lived for over 40 years. Highly recommend!

Our three favorite day trips from Rome were to Tarquinia and Cerveteri for the Etruscan tomb sites there (we rented a car for the day), to Ostia Antica which is a ruin of an ancient Roman city much like Pompeii and Villa Adriana. near Tivoli for the great ruins of Hadrian's summer palace.

There are literally dozens of great day trips from Paris; here are a few
https://janettravels.wordpress.com/category/day-trips-from-paris/
Crecy la Chapelle and Moret sur Loing may be a good choice for your goals; there are boat trips on the Loing to the Seine in Summer.

But so many others e.g. chateau like Fountainebleau and Vaux le Vicomte; small medieval towns like Provins and Senlis; Auvers sur Oise where Van Gogh lived his last days; small cities like Reims (Champagne houses as well as great Cathedral) and Rouen. Monet's gardens at Giverny. Just so much. We try to do a day trip a week when we are in Paris and haven't run out yet. This year we went to chateau Maintenon, and to Nogent sur Marne a couple of times to explore the east end of Parc Vincennes and Nogent and the Marne itself. There are no end of great choices.

Love the Cinque Terre and Orvieto from Rome!

Vernazza in the Cinque Terre is at least 4-1/2 hours from Rome by train and over 5 hours by car. One way. Not a day-trip.

How long will you have in Paris and Rome? You could spend a month in each and barely scratch the surface. If you have a week or less in each, why not throttle back a tad and savor what they have to offer instead of running pillar to post.

Be that as it may, from Paris you could take a day trip to Monet's Gardens. From Rome, day trip to Orvietto for a taste of the hill country or Ostia Antica for some archeology.

From Rome, I would definitely go to Ostia Antica. It is an easy local/suburban train ride away. In fact, if I remember right you can purchase a day pass for the subway and also use it to get to Ostia Antica. We got done touring in mid afternoon and took the train the rest of the way to the city of Ostia. They have a nice boardwalk and we ended the day with a gelato on the beach.

From Paris I would suggest the city of Joigny. This small town in Burgundy is definitely not on the tourist map. But if you take the Burgundy express from the Bercy (maybe gare de lyon I forget) train station, it is the second stop and less than an hour away. It has a nice market on certain days. It is right on the Yvonne river. The old town is on the side of a hill and partially pedestrian only. It has many 16th century half timber buildings and three great old churches. I would recommend stopping at the tourist office, they have some nice walking maps. My wife and I only went there because my great great great great great great great great great great grandpa was baptized in one of the churches in the early 1600's. It turned out to be one of the highlights of our trip.