Just looking at the Reims tourist website and going through a few shops and restaurants and it appears Reims is asleep on Sunday?
What are some suggestion of what we can do in the afternoon/evening?
Not just Reims, France generally, although museums are normally open so it makes sense to plan trips so that you spend Sundays in places with interesting museums.
Otherwise, copy the locals, have a really good leisurely Sunday lunch which could take several hours. Any decent restaurant will be open for Sunday lunch - it is their most profitable time of the week.
Was in Reims this week-end.
Today the souvenir shops around the Cathedral were open. The shop in the Tourist Office by the cathedral, too.
The Palais du Tau (which I would recommend visiting) has a shop with interesting items.
There was a craft market in the Halles du Boulingrin market, which it seemed a one off. However, a book market is held there every second Sunday of the month reims.fr/95-1298/fiche/marche-aux-livres-aux…
YOu can do about anything but go shopping. That shouldn't be a big deal, but don't go there if that's your main goal. The museums are open, cathedral is open, champagne houses are open. Everything is pretty much but major shopping.
If all you care about is shopping and restaurants, I wouldn't probably ever go to the trouble of going there, Reims isn't some major shopping destination.