With 5 hours it is feasible to go to the center by train (RER B), have a look at the eifel tower (metro stop Trocadero is the best place to look at it), then a lunch or a stroll around or something, and go back.
To get to Trocadero would take an hour max, you could hang around for 1 to 2 hours, and then head back. With delays and time getting under 5 hours you could still do it but it would be a rush.
Otherwise. Stay landside at CDG for at least half of the time, and indeed, all there is is a bunch of restaurants and a few not very special shops.