I usally put in my wooden bowl, my tinplate my fork knife and spoon, a book, if available some food (they would have carried bread etc. in there) a few small towels, my box with spices, 3 pairs of spare socks, a spare shirt, my remaining musket tools including some oil for the gun (I have the most important ones including one small towel in my cartridge box) and my gilet vest. If I have space left, I put my tin mug into the backpack too, if I don't I tie it on the back, outside.
Then obviously, put either the greatcoat (whichI cant atm, since I have no habit) or a blanket on top of the closed backpack.
That fills it up pretty well but there is still some spare space left. I need to get more "proper" stuff, like brushes etc. (to clean shoes), a shaving kit (even though I don't trust myself using it to shave, just for the sake of authenticity) I was also thinking of getting another pair of shoes to tie them to the back of the backpack.
I obviously also have my bread bag filled up with stuff (A small pocket knife, food if I have any, another small towel and some candles) and my cartridge box has all important cleaning tools that I might need during battle.