Dear ksc,
you are very welcome.
I think May is a good period of the year to visit Belgrade (not too hot yet).
I have posted some info about outside exhibition of museum:
http://gmic.co.uk/index.php?showtopic=35044
Apart from military museum, if you are in Belgrade it is good to visit Air force museum, which is situated near Airport Nikola Tesla in Belgrade.
I have posted some info about it on this forum:
http://gmic.co.uk/index.php?showtopic=7393&st=40
You can find more info on web sites:
http://www.muzejrv.mod.gov.rs
http://www.jat.com/active/en/home/main_menu/travel_info/jat_review/mart_2007/muzej_vazduhoplovstva_beograd.html
There is also a good flea market in Belgrade, but I am not shure on which days it works, since I do not live in Belgrade.
Have a nice trip.
P.S. We all have the same problems with wifes. ;)