Whilst sailing down the Danube I took the opportunity yesterday to inflict the Belgrade Military Museum on my wife. Situated in a fortress which dates back to the Medieval period,the museum has an interesting collection of WW2 vehicles and WW1, WW2 and more modern artillery pieces. This part is free to wander about, but the artifacts are in the main sadly neglected. The indoor museum costs about £3 to enter, and could do with a bit of a makeover. That said some fascinating artifacts from Roman, early Medieval and Ottoman periods. The 19th century wars are well covered but the bulk of the galleries cover WW2 and the Yugoslav partisans. My lack of Serbo-Croat didn't help my understanding. The final case contained parts from the F16 and F117 shot down over Serbia in 1999. For the money worth a visit.
Also worth checking out is the River Monitor Sava, moored close to the passenger dock. Originally part of the WW1 Austro-Hungarian river flotilla it served in the Yugoslav navy until the 1950s.