If you're visiting Sirogojno Open-Air Museum with your own transport, it's worth making the 4km trip north to this limestone cave with a massive entrance. The underground river that runs through it has created fantastic pools and waterfalls, illuminated by coloured lights to magical effect. The entry fee includes a short guided tour; expect to spend not more than 30 minutes inside.
Note that there are steep walks involved and the cave is not yet wheelchair-accessible. The parking fee is 70RSD.