This depends where you are going, if riding off road I take a spare inner tube and levers incase I can't repair the puncture, plus tools to remove wheels, cable ties,tape etc etc on top of my everyday riding kit. I always have a pump, pepperoni strips, metal putty, all under my rack in a wunderprick bag.Long tours I pack a wifi GS911.
Under the seat i have one of them cheap eBay jump starters, seems to work well.
I always ride with a back pack that has a multi-tool and a few torx, add a phone and a credit card and i'm done.