IMO the minimum you need is:
tube, patch kit, 1 tire lever, money, sm. multi tool, copy of ID, and pump or co2 with 2 cartridges.

I have a very small seat bag and it all goes in there except the co2 pump...it goes in my jersey pocket along with my cell phone, a copy of my car key...no need to carry the whole key ring...and maybe a gel if I plan on going more than 30 miles. I like to travel light on the bike.