Novi Sad is a Serbian city that was born Habsburg. It was founded in 1694 by Serb merchants who settled across the Danube from the Habsburg fortress of Petrovaradin, and for the next two centuries it grew into a trading town that called itself the Serbian Athens. In 1849, during the Hungarian revolution, the fortress garrison shelled it and much of the city was destroyed. What you walk through today is the rebuilt city: pastel Central European facades, a neo-Gothic church tower, a Hungarian-Jewish synagogue and a long river promenade.
It's smaller and calmer than Belgrade, and that's the point. The center is one pedestrian zone, and a baroque patriarchal town and a ridge of old monasteries lie 20 to 35 minutes away. Three days is the right length: one for the old town and its museums, one for the fortress, one for the wine hills of Fruška Gora. Two days works if you skip the monasteries, and a fourth would mostly repeat cafés.
Day one is Liberty Square, Dunavska and the river, day two Petrovaradin Fortress, and day three a day trip to Sremski Karlovci and Fruška Gora. Start on a Tuesday: the Matica srpska Gallery and the Museum of Vojvodina are closed on Mondays, and the city museums usually are too. You're in Serbia, so read the money and registration tips before you arrive. In November sunset comes around 4pm, so each day puts the outdoor part first.