I have two bags on my bike, one an oversized under seat wedge and one a small bag that sits on my top tube and fastens to my stem riser. I put the second one on to help cover up the stem riser which I think sticks out like a sore thumb. Now that I have the small one up front it is very handy.
Seat wedge contents:
tube repair kit, two tire tools, spare tube, band-aids, usually ~$10, a small pocket knife (for peeling fruit), aspirin, mini tool, spare chain pin ( I quit carrying my chain tool so I don't know why I still have this). Plenty of room left over for an apple, rain jacket, etc. as needed.
Top tube bag: 1 or 2 granola bars, butt butter, and usually my glasses. (also a great place for the camera)
I also took a couple pics of my bike showing the Road Morph G pump that I carry, the rear blinky light (Bell) and the stuff on the bars which includes the mount for my head light, my speedometer, and my Incredibell. I usually only put the light on as needed when I plan properly. I probably carry more than most people, but I feel comfortable with what I have on the bike. I also always carry my cell phone. I clip it to the strap on my seat wedge and if it starts raining I put it in the seat wedge.
Here it is ready to ride.
Here is the view from seat. (the front brake cable was too short when I added stem riser)
Here is the contents of the seat bag
Here is the contents of the top tube bag (not the light)