I don't suppose there's much chance that you have lots of time, but both Timisoara and Belgrade are interesting cities. If you're willing to spend a lot of time on buses, you could do worse.
I traveled from Zagreb to Timisoara by bus in 2015, with a 2-night stop in Belgrade along the way. Novi Sad's also interesting. And later I bussed from Sofia to Podgorica, Montenegro, with an overnight stop in Nis, Serbia.
I'd do both trips again. It does require some flexibility of schedule since it can be difficult to pin down bus schedules with certainty.