I was in Reims earlier this month and stayed at the Best Western Hotel Centre and also took the Mumm tour.
The hotel was perfectly fine and the staff was friendly and helpful. It is located right in the middle of the Place Drouet d'Erlon, which is very busy in the afternoons and evenings with lots of restaurants and bars and a young, college student-type vibe. We were there over the weekend when France won a match to advance in the Rugby World Cup, so we could hear some noise from the plaza that night. We enjoyed the location and given the circumstances, the small amount of noise was not bad at all.
The Mumm tour was enjoyable, with a short video and then a group tour of the caves, which included quite a bit of information about the way champagne production has changed over the years with some displays of equipment from different eras. The tasting at the end was basically just a glass of champagne that we took out to their nice patio area. We did book the cheapest option for the tasting, so I think other people may have gotten more of a guided tasting with either a more expensive champagne or maybe more than one. Overall, I thought it was worth doing.
We also took a van tour with Cris-Events (mentioned in the RS guidebook, but the website and name seem to have recently changed to A La Francaise, https://www.alafrancaise.fr/en/). We did the half-day "Afternoon Epernay & Family Growers from Reims" tour. It was a little pricey, but the experience was great and we got to visit two smaller vineyards and in one case chat with the owner/wine maker. It made an interesting complement to the Mumm tour and included six tastings, so it was great to have a driver! We came back with three bottles and had a wonderful afternoon.
As for Reims itself, don't miss the Halles Boulingrin market. We took a picnic of things we bought there to the lovely park Les Hautes Promenade.
I hope that helps. Enjoy Reims and your tour!