For a destination trail run, drive 30 minutes southwest of Novi Sad to Fruška Gora National Park. This large national park surrounds Mt. Fruška Gora, and is quite undeveloped, with a single road and no visitor center. There’s a good number of hiking trails, and a nice variety of terrain: fields, orchards, forests, vineyards, and 16 medieval monasteries!

Generally, this is moderate to challenging trail running/hiking. This page has excellent information about the various trail & loop options. There’s a map and GPX files that can be downloaded.