Belgrade’s Nikola Tesla airport has a small business lounge airside.
When you go through security you come out facing the duty free shop. Turn right, and head towards the “A” gates. The business lounge is located near gate A5.
The lounge has quite comfortable deep armchairs and friendly staff who speak English.
Wifi is available, but they block a lot of services, so it’s only useful for basic web browsing and checking email.
Unlike most airport lounges that I’ve been in there is no “self-service”, so you have to ask the staff for any drinks or other refreshments that you want.