We will be in Reims as a day trip from Paris, arriving via train. I haven't been able to confirm that Uber/Lyft are operating there now, so am wondering what other modes of public transport will give us the most flexibility to visit a few champagne houses in a day.
Bus and trams. Day ticket is 4 EUR.
http://www.reims-tourism.com/Prepare/Practical-information/Move-in-Reims/Move-in-Reims
The larger houses with tour centers/caves in Reims offer tours at set times. Between the tour/tasting they take a little while, so if schedules align perfectly I think it might be challenging to do more than two in a day, especially if you're planning to see any other sights in Reims. Some of the houses take walk-ins if they have room, but most require reservations. There is a bus that picks up right in front of the train station and goes very near to Martel and a short walk from Veuve Cliquot (not sure of the bus number, ask at the very helpful tourist information center just to the right side of the train station circular driveway). Likely there are others close to the bus/tram as well; those are just the two that I know of off the top of my head.
If you're interested in going further afield for a half-day tour, there are lots of well-regarded little tour operators (we've liked Cris-Event) who take small groups in minivans directly from the train station to visit several small producers in Hautvillers and elsewhere. Whichever route you choose, you'll need to book online in advance.
Enjoy!!