We are first-time everything, but we have 2 days between the end of our river cruise in Strasbourg and our flight leaving from Paris. So ... it SEEMS that we might make the best of our time seeing Strasbourg and the area for a half-day/night/morning, then going to Reims for a day/night and into the airport to catch the flight.
Less is more. Especially for first-time travellers that tend to underestimate the elapsed time in changing hotels. Every change is going to take about 1/2 your sightseeing day if you add up all the activities; you need to pack and check out, get to the train station in advance of your train, buy tickets, arrive in Reims, go to the new hotel and check in, go to the new room and perhaps freshen up a bit. And as first-time everything travellers you should plan for extra time to figure out how to use the ticket machines or to wait in line to buy them, read the signs and figure out how to find your train track, etc. By the time you are out and about in Reims it's going to be 2pm or later depending on what you mean by a morning in Strasbourg. It really isn't worth it, IMHO, but since you talk about "make the best of our time" perhaps a few hours sightseeing in 2 cities is right for you.
I'd suggest picking either Reims or Strasbourg for the 1st nite and then going either into Paris or to an airport near CDG the 2nd nite. If you leave early/mid afternoon the 2nd day then you have time to get to a hotel in Paris, walk around a while, have a nice dinner, and it will be easy to get to CDG for a 2pm flight. Given that you should be there 3 hours early and you should allow an hour or so to get to the airport, even though the flight is at 2pm you don't really have free time that morning to do any sightseeing. If you choose to stay for dinner that 1st nite then I'd just go to a CDG-area hotel; you could go to Paris instead but by the time you get to your hotel it will be pretty late.
We've been to both cities and like each. Both have a great Cathedral, pedestrian-only area, nice shops, etc. You'd have a few hours more time sightseeing if you stay in Strasbourg so that's what I'd do unless you have a great interest in champagne.
edit: for the TGV you save if you buy the train tix in advance, so one less thing to do on your travel days