A visit to the park and the National Archaeology Museum in Saint-Germain-en-Laye.
Why not take a walk in Saint-Germain-en-Laye? Saint-Germain-en-Laye is served by the RER A (at the very end of the line) and is therefore very easy to get to. The walk starts as soon as you get off the RER. Then you can visit the castle and the park, the castle contains the museum of Archéologie nationale, it is a very classical museum with many objects exposed in the windows. The entrance is paying but you can enter the courtyard of the castle and visit the chapel for free.
Then you can walk in the park and especially see the Great Terrace which offers a magnificent view of Paris and its surroundings. The castle and the park are next to the forest of Saint-Germain, so you have many possibilities to walk around. Here we are satisfied with a rather short walk.
To sum up, it is a very good idea for a Sunday afternoon walk.