>>512853458
Swimming, water skiing, kayaking (hopefully not in rivers), going to raves, kafanas, other kinds of clubs, visiting museums, galleries, churches, medieval fortress and underground tunnels, exploring Austrohungarian/Turkish/Brutalist/pseudo-Dubai BW architecture, sunbathing, riding bikes, hiking in multiple forrests, going in boat trips, gambling in big cazinos etc. etc.
I don't think people appreciate how fun Belgrade is for being such a messy shithole until they to to a more "decent, ordered, silent" place and want to claw their eyes out of boredom
I visited few museums/churches in Sofia, walked around the (much nicer kept) center, went out of my way to visit (much less interesting) commieblocks and toward their mountain
But that's essentially all there is to do. Clubs are small and boring, nightlife is nothing compared to Belgrade, you have to go out of your way to find anything interesting to do. Zagreb is even worse but at least it can say its because it's small, Sofia is a relatively big city