Agree with Pavel. But, if you don't have a 6 or 12 pack, might as well use that space for other stuff.
First I try to put the most dense (heavy for its volume) stuff closest to the bracket because the bag has less tendency to sag that way. Also, the closer your weight is to the headtube, the less that your handling is impacted so the densest stuff should be closest to the headtube.
I tour with Ortlieb panniers. They are not very convenient to go into to grab something that you want during the day, so the stuff in my handlebar bag is pretty much anything I might want during the day. (But bike locks are heavy, put them elsewhere.) Also, if I stop to go into a grocery store or restaurant, the handlbar bag holds my valuables and small stuff I really do not want to lose, so I can quickly and easily take all my important stuff into the store with me.
In the photo, the pocket on the left side has my bag shoulder strap, multi-tool, maybe one or two more small items for the bike. The pocket on the right held my camera and spare camera batteries. Sometimes also have a bandana in that pocket. There is a pocket on the front of the bag that you can't see because it is under the lid of the opened bag lid, in it I have first aid kit, bar bag rain cover, maybe other stuff that I rarely need but I want it easily accessible just in case I suddenly need it.
Inside the bar bag I had my smartphone, sunglasses, a AA rechargable battery charger powered by my USB charger that was powered by my dynhub (the square white thing with wire sticking out of it), some other electronics and cables such as a Lithium Ion battery charger (not currently in use), a bunch of granola bars and protein bars, rain cover for my saddle, GPS that was not turned on, tooth brush and toothpaste, a day or two of meds, some small guidebooks (they are dense, are oriented vertically against bar bag bracket), a couple small Li Ion powerbanks, and I am sure there is some other stuff that I can't see in the photo. I probably had my spare money, credit cards, passport, etc., inside of it as these are the valuables I do not want to lose. I had a small wallet in my pocket with the stuff for immediate needs for shopping, so the bar bag would have had the more long term types of important stuff like passport and additional credit or debit cards in a ziplock bag.
On the trip where I took the photo, my spare tubes, patch kit and tire levers were in a small bag on the frame, so they were not in the handlebar bag. My sunscreen on this trip was hanging from a clip elsewhere, thus not in the bar bag.
In the photo under the bar bag you can see I had my yellow frontloader panniers, the red thing strapped on top of the right side pannier is my rain jacket where it is handy and quickly accessible, on top of the left pannier was my rain pants and rain shoe covers. Thus, this rain gear was not in the bar bag.
Second photo shows the map on top.
Weight in the bar bag will adversely impact your handling, so only put as much in the bar bag as you are comfortable with. I had enough stuff in this bag that I could feel it on my handling, but I was comfortable with it so it was a trade off of handling vs convenience.
ADDENDUM ADDED LATER.
In the photos you can see my Rohloff shifter. You might have to resist the temptation to run out and buy a new Rohloff bike.
I have since moved the shifter to the right handlebar end on a HubBub adapter. I like the new position much better.
