I keep a couple of granola bars in the cantle bag. I add a fresh bottle of water each ride. There is usually a knife and a space blanket in the cantle bag.
Depending on the day and where I'm riding, I'll add a rain slicker, apple or string cheese for some lunch, my gps and maps if I'm riding some place new.
Cell phones don't work much where I ride, no signal, So I don't bring them unless I know they will work. A small camera is usually in my shirt pocket.
Longer rides and wilderness areas I carry a 41mag in the holster. And often I bring a flyrod and some flies if there is a place to fish, usually on wilderness rides.
During hunting season, I throw some cotton bags in, so I have some place to put the meat, I'll add a rifle scabbard to the saddle to carry my rifle, and depending on where I hunt, I'll roll up some saddle paniers and tie them on behind the cantle. When we kill an elk or deer we cut it up and throw it in the panniers and lead the horse back to camp.
Of we are staying the night, I bring along a pack horse.
Also, if it's going to be an all day ride I bring hobbles for the horses, so I can turn them loose to graze while I eat lunch