I have always carried;
1-2 bottles on bike,
mini pump on bike,
topeak 18+ multi tool with chain tool,
3 park tool levers, (never know when one could break,)
2 tubes, 1 for mountain biking, if I puncture I'll buy new tube en route to keep my compliment up,
pack of self adhesive paches,
a few cable ties,
small cave lock,
two spare rubber straps for my light mounts,
little bag with 2 each of 9,10 & 11 speed quick links.Ready for all my bikes.
Keys.
All in a topeak aero wedge saddle bag with room to spare.
Not really a lot to look at to cover most eventualities.
Edit to add phone and money and card in pocket.