When I had one? 2 pairs of socks, 2 pairs underwear, one spare uniform, sweater, raingear, mess kit, spare canteen, sleeping bag, mattress, shaving kit, and most importantly, 4 spare magazines for my FN-C1.
Sometimes, a good book, and spare batteries for my flashlight.
Ahh, the good old days....

Dean.

Hah! I saw one, used in my country's army.

they have socks, another set of uniform, spare boots, mess tins, and some other items.