This is a very interesting topic, especially if widespread WFH becomes a permanent aspect of our society.
A few things that I have seen work pretty well that do not revolve around retail. I realize most of these aren't going to fill the same space as ground-floor retail, but they do activate the street. Sidewalks should be treated as travel lanes, but you need the equivalent of roadside stops or attractions to keep people there. Most of what I propose would be suited for plazas, parks, or possibly parking lots. If you go to a place like Manhattan, you'll notice that the sidewalks are very active, but that's mostly from people going from place to place, they aren't stopping (and if you do stop in the middle of the sidewalk, you'll quickly irritate every New Yorker in the immediate vicinity).
1. Interactive art installations. (or funky art installations like the Bean in Chicago)
2. Free Outdoor theatrical performances (a la Shakespeare in the Park)
4. Splash pads, misters, and other cooling features
5. Swings, rocking chairs, and other nontraditional seating.
6. Put the ACC tournament on a big outdoor screen (I haven't seen this, but think it would be fun). In Durham, specifically, you could also put the Duke-UNC games on a big screen and have local retailers set up kiosks.
7. Have a to-go beer zone in downtown (like they have in Savannah).
8. Summer beer gardens
9. Free Outdoor yoga/group exercise