All of those things, really. I use the smallest Specialized seatpack to carry: tube, patches, tyre levers (2), chain tool, chain link/pin, CO2 inflator "head", 2 x CO2 cartidges. The pack is quite full, but it can be cinched up close to the underneath of the seat. This also stops it rattling – a key requirement!
Mini pump (Lezyne, natch) goes on the frame. Phone, bank card, ID, cash, inhaler, food in jersey pockets.
I wouldn't take a block of Soreen with me, though I do know an ex-1st Cat who usually sticks one in his jersey pocket….
you'll be reasonably prepared if something happens rather than relying on phoning someone up and asking them to come and get you.
Indeed, like the bloke we met at the top of a rather windy Cat & Fiddle on a club run last autumn. It was nice and arm in Macc, so he'd ridden up in his Rapha jersey and shorts on his lovely titanium Omega, and was so cold at the pub at the top he'd called his old man to fetch him..!