Paris

Before the questions can be answered, need to know the airport you are using.

...and how long your layover before your connecting flight.

Oh, right! Charles de Gaulle for about 7-8 hours.

Effectively the RER works the same as the Metro and connects to all Metro lines. Just take RER Line B into centre ville.

I agree with Pat. With a 7 hour connection time you cannot possibly leave the airport and go into Paris "for a few hours". You might manage 45 minutes if everything works for you. Please consider that a) your flight may land in Paris later than scheduled. b) you have to go through Passport Control and unless you have an EU passport this is a lengthy procedure. c) you have to check back in for your connecting flight 2 to 3 hours ahead of departure time AND you must go through Security when you return to the airport.

I wouldn't do it with a 7 hour layover, might think about it with an 8 hr one.Will your luggage be checked through to Istanbul? If not what will you do with your bags?

If luggage is checked through you may be able to grab RER into Paris( about 35-40 minutes), get off at St Michels, wander around Latin Quarter for a half hour or so, wander across bridge to Notre Dame, visit inside church( but not Towers, that line is too slow, line for church looks long but moves quickly), take metro to ET,, visit, ( likely not go up,, lines ) and then back to airport..

I have not done this,, but do be aware you will have to go back through security,, so you should aim to be back at airport at least 2.5 hours ahead, many would say 3 hours..
So, your 8 hours, minus 80 minutes( lets round it up to 1.5 hrs) on RER, minus 2.5 hours, leaves you 4 hours in Paris.