3 days in Nis

Hi, I will be arriving in Nis for 3 days in January,  I've never been to Serbia before so I don't know if it is better to stay in Nis sight seeing or stay somewhere else and maybe do a day in Nis, If I travel it will be on public transport, also I will be flying back home from Nis so I can't really travel too far from Nis. Any ideas please....Thanks Tony