Seine cruise, no dinner, Vedettes de Pont Neuf, about 15 euro pp, one hour, live narrated cruise, leaves from western tip of Ile de la Cite at Pont Neuf. Can reserve a time ahead at their website. Sit up top for best views. We have been many times and find it's always worth it. Nighttime try for late enough to see Eiffel Tower and buildings lit up. Daytime great views along the route also. Depart and return at same spot. No disembarking partway through. Great bargain also! There are options for champagne cruise, etc., but I feel they aren't necessary, and I believe they are only on the lower level, which isn't as good for views of Paris. Get there about a half hour early to snag a top deck seat. Bring jackets if weather is cool.
Aside from the Musee d'Orsay on the left bank, the Cluny is a favorite museum of mine, full of medieval art and artifacts in an old mansion for upper echelon clerics, recently redone and well worth a good three hours. Walkable from the Orsay. Also, but not in there same area, the Rodin Museum, especially the sculptures in the garden, is terrific. I visit it every time I have been in Paris, it is such a treasure to me.
Also in St. Germain area for a reasonably priced meal is Breizh Cafe (there are 3 or 4 in Paris) with delicious Breton crepes both savory and sweet. I have seen mostly Parisians there (lunch time...on their lunch breaks from nearby businesses, or so it seemed to me). Excellent food, good prices, and distinctly French (from Bretagne, of course).
For a stroll, you can't get more Parisian than walking along the left bank right next to the river, checking out the bouquinistes (sellers of used books, bric-a-brac, posters and prints, etc.).
Amusez-vous bien!
