Paris

First, I would probably get a Navigo Month pass for all zones. Then you have Paris and suburbs covered. That's 113 euro. If you just the inner zones (Paris proper) its 65 Euro. The rest I would take a la carte.

The inner pass for Paris is probably a good idea (Zone 1 and 2), not sure it makes sense to go for larger. For your trip outside of Paris, you may think of buying a Railway card http://www.voyages-sncf.co.uk/best-fares.html
enjoy!

Hi, For both Amiens and Reims you can take the TER regional train. Buying the point to point ticket is good enough. You leave from Gare de l'Est for Reims and Gare du Nord for Amiens. I don't know Reims as well as Amiens but I really like Amiens, a lovely town to walk around. If you want to see a town without tourists near to Paris and Amiens, go to Beauvais. The cathedral and military cemetery are interesting and worth seeing.

Well places like Chartes and Riems are not covered on any pass, they are point to point tickets , and can be pretty cheap if purchased ahead on sncf.I just did a dummy booking for July 8th and got a return ticket for 24 euros . A monthly zone 1-2 pass may be worth getting though for some people, I personally have spent up to three weeks in Paris and used only 2.5 carnets( a carnet is just 10 one way tickets good for zones1-2 for bus or metro, sold at a discount, 13 euros) , since I like to walk a lot and stay centrally I find I am not taking the metro as much as some people do.. its a personal preference. I find the most interesting things by walking.

Chartres is a regional train too. These are all within commuting distance for many people. If you are 60 and over, you can have a 25% discount at the SNCF (train) ticket window.