9hour layover in Paris. Can I go out of the airport?

You probably can and taking an hour to get to Paris either gare du nord via RER or Roissy bus to Paris. Then you probably have like 5h to visit before headin back to the airport. You probably can see major attractions but I don't know how busy it gets specially if you plan on climbing the Eiffel Tower. You could go visit it then hit Latin quarter to visit notre dame, Sorbonne. Go to place de la Concorde, opera and department stores then have lunch somewhere and take the RER back to airport. Easier and faster compare to traffic on the road.

9 hours is more than enough to ge a glimpse at Paris. You can book some fast pass for the Eiffel tower, Louvres or a quick 2h tour in a bus.

It takes 45minutes by train to go downtown from the airport.

If you leave Paris from another terminal or another aiport (Orly), I would recommend you to use a mobile luggage storage. You can get back your bags right in front your check-in desk so you will save some time during your layover.

There are many fast pass companies, I do think it it important to get one as some days, it can be crazy how huge waiting lines are.

have fun

Since France is part of the Schengen Area, you'll pass Schengen immigration when you land in Paris, and your flight to Prague will leave from the Schengen zone in the airport. Therefore, administratively speaking, you'll be in France, and you can wander around wherever you like.

Time-wise, count on roughly one hour for immigration, one hour each way for transportation, and one hour for security checks and boarding. This means you can plan to spend about 5 hours in the city.

Regarding transportation, the fastest option at peak hour is the RER B train, and the fastest option off-peak is a taxi. The difference isn't much however, so there's no reason to take a taxi unless the trains break down (which happens occasionally, but rarely so off-peak — and these days there are occasional taxi strikes, so beware). The trains can be very crowded at peak hour, giving an advantage to the airport buses, but the buses are slower, so for you the train is the best option. At the airport, follow the signs for “Paris by train”. On the way back, make sure you board a RER B train that goes to “Aéroport Charles de Gaulle — Roissy” and get off at the right station (there's one for terminals 1 and 3, and one for terminal 2).

You can get a tourist day pass (Paris Visite zones 1–5) covering the airport and Paris for 23.50€. (There's a cheaper day pass but it doesn't cover the airports.) A ticket between Roissy airport and Paris (any metro station) costs 10€ each way. Get tickets if you plan to stay around a single place; get the day pass if you plan to use the metro or the buses to see multiple places.

What to do obviously depends on your tastes. You have time to visit a monument or two, but beware of queues. February is off-season, but still, if you're planning to visit a famous place like the Louvre or the Eiffel Tower, I recommend booking tickets in advance and looking up advice about times.

Do plan to get something to eat in a bakery or café, that's part of the Paris experience. If you want to save time, it's unexceptionable to eat on buses and trains in France (just don't spill and don't pick extremely odorous food). However I don't recommend getting food at the airport, it'll be expensive and low-quality.

Yes with 9 hours, you have way enough time to get to Paris. If you arrive mid-day for instance, it will take you 1h to 1h30 to get out of the plane, get through the passport control, take your luggage with you and then reach Paris. If your plane arrives during busiest periods (morning between 7-10 AM and afternoon between 5-8 PM), then you might need more time to reach Paris if you take the bus or a taxi.

I advise you to arrive at least 2 hours before the flight in CDG. So this leaves you around 5 hours to visit and enjoy Paris. In 5 hours, you can do a lot of things. So it really depends on what you like. 5 hours isn't enough to visit the whole Louvre Museum but it is enough to visit the Eiffel Tower. Also way enough to do some shopping around the Champs Elysees area...