You also might consider a day trip to Angers. Take an early TGV from Montparnasse to Angers. The train station in Angers is in the center of the town and only about three blocks from the Château of Angers. There are some beautiful gardens in Angers and the old section of the city is interesting to walk through. Everything is within walking distance of the train station and you could then take a afternoon train back to Paris. The train ride is less than two hours.

I will second (or third?) the Army Museum located at Les Invalides. I'm not really interested in that sort of thing, and it was truly amazing--they actually have a real taxidermied (is that a real word?) horse in full armor for example. Your son being a history buff, I think he would have a great time.

Just a few more days until we leave. Again I wish to thank all of you for your wonderful ideas.

I may have missed it but I didn't see the basilica at St Denis mentioned.

Since your son is into history it's where a lot of French royalty is buried.

Next to the last stop on the north bound blue line #13 metro.

I haven't been yet. It's on my to do list. Other posters have commented favorably on it.

Please be sure to give a trip report detailing your son's favorite activities. I have read this thread with great interest as I will be in Paris and Normandy for two weeks with my 14 yr old son in June.

For lunch there are any number of casuale cafes - mostly with in and outdoor seating where you can get very moderate meals - anything from a croque monsieur on up.