Looks great Chris.
These baby's usually had a tarp cover for the russian weather, so a rolled up tarp would be one clear suggestion. Then how about duffel bags, tent pack, blanket rolls, etc. Seeing as the vehicle is so open, a nice touch is a box with cooking utensils in it, ie, frying pan, saucepan, coffe pot, big spoon or ladle, etc. The box can be an old ammo crate or make one from sheet and strip styrene like the utensils. For the frying pan, cut a disk of styrene and add a piece of an old PE fret around the edge for the pan lip. Then make the handle from plastic strip (if you put the handle over the join in the PE it will help hide the join and make it stronger at the same time.
I did this for one of my old dio's and it was surprising how much attention it got for such a small part of the overall thing.