We visited Belleau Wood a couple of years ago and found it very worthwhile. Compared with the Normandy sites, this WWI site is very quiet, which was moving in its own right.
I would combine the visit with a night in the Champagne region, either Reims or Epernay would work fine. We stayed in Chateau Thierry at the Ile de France hotel, which was perfect for us.
In addition to the battleground site which has some rustic trails, there is a an adjacent cemetary at Aisle-Marne, and a large Memorial (Chateau Thierry Monument)nearby. There is another American cemetary (Oise-Aisne) nearby, which you may also want to visit. Canadian and German cemetaries are nearby as well. The American sites can be researched at the American Battlefield Monuments Commission website (the cemetaries and monuments are separate areas of the website). It contains information regarding directions and visiting hours. This site will also provide information for your Normanday visit, if you are planning on visiting the war related sites there.
One night for Bealleau Wood is enough, but a car is required. The driving is easy as the sites are close to the autoroute. We self-guided through the sites and did so using our Frommer's and the excellent road signs pointing to the American sites. It would be a very full day if you tried to make it a day trip from Paris if you had to arrange for a rental car. I think it would be possible as a day trip if you were staying outside of Paris and already had a car. We rented ours from the train station at Disneyland Paris - a great access point.
Someone posted a similar question a few months ago. You might want to check to see if they received any replies other than mine.
