Holiday with chidren (Belgrade)

Any suggestions as to where, within Serbia but outside Belgrade, I could take my 3 children (all under 8) for 3 days ish during the October school holiday. Prefer somewhere within a couple of hours drive.

Already been to Palic/Subotica, Novi Sad/Petrovaradin (+en-route non-motorway locations), Zlatibor, Tara, Golubac (and along Danube to and past it), and Smederovo. Will go to Kopaonic when the snow arrives.

Any other suggestions of interesting places (for children as well)?

thanks

Neil