Belgrade and Roman Ruins/Museums

[Longovicium]

I am spending seven days in Belgrade (Singidunum) in the middle of August this year, visiting a friend's art exhibition, and wondered if anyone could recommend any specific museums or ruins that I could visit whilst staying in the capital?

We will be renting an apartment and can travel outside Belgrade to other sites if they are reasonably nearby. Any suggestions would be welcome! I will of course take pictures so if anyone desires specific shots let me know andI will attempt to fulfill those requests.

Thanks in advance!