The Chinese in LeBourget is really bad, sorry meant to say earlier however, the sushi restaurant (restaurant Japonaise) is very very good. Always packed at lunchtime though.
There is a Pizza restaurant on the right by the roadworks on the way back to the Marriott which is also excellent. (this is not the one near the hotel though that is good).
The Brasserie in Roissy village is very nice. A lot of restaurants are closed on Mondays in France, and between 14:30 and 19:00 most are closed.
If you go into Paris from the hotel get off at terminal 2C or D for the RER. When you come back though try and get off at terminal 1 for the shuttle. The trick for not waiting too long for the Marriott shuttle is get the blue Mercure hotel one if it comes first and walk the 100 metres back to the Marriott. The shuttle drivers don't care where you are going, the locals often use them.
The hotel restaurant is very very expensive. The Holiday Inn opposite the Millennium does nice bar snacks
