There are many “philosophies” here. Some people like to go light (“what can I get away with?”). Some people like to have every eventuality covered (“sure I have a spare bottom bracket, it’s going to cost you”). In general it is a case of personal risk management, you will have to weigh up many factors.
If it is a long ride (into/over-night) you will need reflective vest, lights (see Safety Recommendations) plus potentially warm weather gear, arm/leg-warmers, rain/wind-proof, gloves, clear glasses. Met-Eye is a very useful resource from the Bureau of Meteorology for accurately predicting overnight conditions.
A power pack for recharging your phone/GPS. Spare lights, batteries.
A typical minimum set of tools may include:
Additional tools to consider include:
Gels, figs, banana bread, peanut butter & honey sandwiches? yes, yes, yes and yes. Audax is all about eating. If there is an issue with water or the distance between bakeries on a ride, the RO will likely have pointed this out and you would be wise to take notice. Sometimes in hot weather, two bidons are not enough.
Don’t forget your brevet. A zip-lock bag and a pen also can be useful.