So I've got my lovely long layover in Paris, but I don't particularly just want to stay in the city. (I've lived in China for the past year & half & have had my share of people.) I'd really love to see some French countryside & nature as well as the Eiffel Tower. I'll be flying in & out of CDG & will probably get a hostel near the airport, but is there a National Park (or something else not so touched by mankind) that I could make a day trip out of? I won't be renting a car so trains or buses are my only option...
Paris is a big city (obvious) surrounded by suburbs and then by agricultural land. To find a National Park, you need to go to the far edges of France (Alsace, Alps, Pyrenees, Brittany). Much too far for 30 hours.
And, since you also want to visit the Eiffel tower, that just leaves time for a short day trip.
My suggestion is to hire a car for one day from the airport, and drive out to the Champagne vineyard area. It is the same side of Paris as Charled de Gaulle airport. It is a man-made environment, but green, pretty and easy to drive around. You will need a good map (Michellin 1:200000, anything smaller does not show all villages or the roads between them), and do research and plan your route beforehand.
Perhaps Chantilly would fill your need for open county. It is a lovely castle surrounded by forest and it is a short drive north from CDG. Alternatively, there are some huge parks in Paris.
You have time for a pre-booked visit to the Eiffel Tower ( or maybe you just want to see it, not visit it?) and then a few hours in the Bois de Boulognes, plus a meal of two. You do, after all, have to sleep somewhere during the 30 hours, as do the people who run restaurants, tourist sites, etc. Not everything will be available during your 30 hours.
PS. Don't go to the Bois de Boulognes at night. Not dangerous, just not savoury.