Since I haven't had any major actions lately, here are some things I've been working on before discovering the diary.
Students' Park: http://www.openstreetmap.org/?lat=44.819531&lon=20.458158&zoom=18&layers=B000FTF I think it looks very nice, especially compared to how it looked before: http://cassini.toolserver.org/tile-browse/browse-en.html?lat=44.81946&lon=20.45818&zoom=18&layers=B I would add benches and trashcans, but the renderer won't display them so I haven't bothered yet. To the right, a street passes under the building of the Faculty of Mathematics. It seems that there is no "official" way to map such an arrangement, so I mapped it as a tunnel.
Pedestrian passages around Knez Mihailova, and the more recent Millenium Shopping Center: http://www.openstreetmap.org/?lat=44.816647&lon=20.458094&zoom=18&layers=B000FTF I think that even most Belgraders who don't live in the center don't know about them. If you've parked your car in the garage below, and have your business in, say, Marka Leka street now you know you don't have to walk around, you can go straight through the buildings.
Pedestrian underpasses in front of Palace Albania and near Belgrade Youth Center, and Čumićevo sokače shopping center: http://www.openstreetmap.org/?lat=44.814911&lon=20.461296&zoom=18&layers=B000FTF While not very big, this is the most extensive corridor system in Belgrade that I know of. I have dutifully marked the underground toilets with tunnel=yes, the renderer doesn't display them any different than overground toilets, which has some potential to cause trouble for people who might need them...
Also, a few more pedestrian underpasses, which covers all of them in the city center: http://www.openstreetmap.org/?lat=44.812845&lon=20.461157&zoom=18&layers=B000FTF Given that some cities (Paris, I'm looking at you) have extensive labyrinths of underpass systems one could easily get lost in, connecting the surface to metro to shopping centers to other places on the surface, where are the maps?
And of course Belgrade Fortress and the Kalemegdan park: http://www.openstreetmap.org/?lat=44.82371&lon=20.45076&zoom=16&layers=B000FTF Not even half-finished, but will have to wait for GPS.