We did a day trip from Paris to Champagne a couple of years ago. We took the TGV from Paris, Gare de l’Est, to Reims in the morning and returned to Paris that evening. As Rick noted, it’s maybe a 40-45 minute train ride, with speeds topping out at 300 KPH. Once in Reims we walked from the station to Ruinart for a morning tour and tasting. It’s about a 30 minute walk through the city center and some neighborhoods. After Ruinart, we wandered back towards the center of town and found a very casual restaurant for lunch. We walked back to Taittinger for our afternoon tour and tasting. It was a similar 30 minute walk back to town, where we visited the cathedral before catching our train back to Paris.
To be honest, I don’t know what a driver or organized tour could have added to our day. I could see how they might be helpful if you wanted to visit other houses or venture outside of Reims, but neither was part of our plan.