To enjoy day trips from Stockholm will have to depend on the destination you are to choose and on the season you planned to have the trip. Say, you wanted to be in Mariefred in the morning and to head back in the afternoon. On summertime you can go one way with the steamboat or with an old locomotive that runs between Mariefred and Laggesta or if not a train, in less than an hour. And also, if you will prefer, you can take a ferry in front of the Grand Hotel to Vaxholm which is about 30 minute ride and stops along the way to pick people up and drop them off. But, actually when you are leaving Stockholm there are a number of day-trips to choose from considering the location in the water.
It might be better to keep your options open for any of the multi-day trips to make itineraries easier and less complicated along with possible all-inclusive packages you might want to avail so that you can see all the sights in a limited time. For example, you might intend to spend two days in Normandy and Brittany to discover the Landing Beaches, the city of pirates Saint Malo and the Mont Saint Michel Abbey. Your itinerary and activity schedule on these trips will include having the Normandy motorway, a first stop at Rouen for a walking tour to watch Gothic structures, churches, museums, and to be on the site where Joan of Arc was burned at the stake, then continue to pass by the Landing Beaches of Omaha, dinner and overnight accommodation at the Novotel Caen, set off the next day from Caen to Saint Malo in the heart of the Brittany, after probably lunch time depart for Mont-Saint-Michel to visit the abbey, stroll and shop, before finally heading back to Paris.
Another option may be a pilgrimage to the town of Lourdes, noted for the miraculous grotto of Massabielle, the healing baths, and where in 1858 the Virgin Mary appeared to a14 year-old peasant girl named Bernadette Soubirous eighteen times. Here you start with a transfer to Montparnasse train station to board the a train that will cross the Beauce, Touraine, Poitou, Aquitaine, Bordeaux, and Les Landes regions of France to arrive at the foot of the Pyrenees mountains, then to the La Solitude hotel from where you can have easy access to different religious sites and other attractions, such as going to the Miraculous Grotto, the Basilica, to the Baths, and to see the impressive day and night processions. And, on the second day you can discover the charms of Lourdes before you will have to board again the train to Paris Montparnasse Station.