I will arrive CDG @ 11 am and have a flight to Aberdeen @ 8 pm, Is it enough time to take a round in Paris?

If you can check your luggage through to Aberdeen you can spend a few hours in Paris. Be sure to be at CDG 2 hours ahead of your flight. If you have to recheck baggage then be at CDG 3 hours ahead of your flight.

Take RER, buy return ticket at CDG and get off at St Michel, a direct run from airport. From there, easy to fill your time walking this area. When you exit station, you are at Seine, with Notre Dame in your sight. The river, the Islands, booksellers,maybe walk along st Germain, Pont des Arts, etc..

I am unfamiliar with a return ticket sold by the RATP. The one way fare is 9.10€ from CDG to anywhere in Paris including any required métro connection. You could go into Paris and return for 18.20€ or if you might want to make a least one additional bus/métro trip you could purchase a one day, zone 1-5 Paris Visite for 19.60€ good for unlimited travel anywhere.

The RER B takes about one hour to/from CDG so allow plenty of time to make your flight. Note: when returning to the airport, be sure to take the RER B3 to CDG as there is another RER B (RER B5) which departs from the same platform but goes to Mitry-Claye, not CDG.

Sarastro, correct. Return RER ticket = 2 tickets. And yes, returning to CDG double check the end point.

Your RER ticket will allow you to transfer to the Metro at station where the RER and Metro cross. So you could, for example, get off the RER at Gare du Nord, transfer to Metro and go to...Champs Elysee.

Getting off at St Michel allows you to see things with no need to hop on Metro.

I'm thinking you probably have...4-5 hours?

I believe St Michel is closed for repairs. Or has it re-opened?

RATP site has no work notices. I do remember someone here mentioning that.

Thanks all for your kind replies, it really helped