I use a small under the saddle bag for tools, repair kits and inner tubes.
I prefer the type which just use velcro straps, rather than fancy quick release brackets. With the velcro type, you can easily switch from one bike to another.
For the waterproof jacket, or rather a rain jacket, I use an old water bottle with the top sawn off. The rain jacket is rolled up and inserted into the bottle and goes into one of my bottle cages. But I always check weather forecasts and if heavy rain predicted, would probably put off until the next day.
Camera or rather my phone (and wallet) - back pocket of my cycle jersey/jacket
Sarnies - Usual rides would not take me through a meal time, so don't need to carry sarnies. For rides approaching 3hrs, might carry a breakfast bar or some fruit and these would also go into one of the back pockets. As would an extra inner tube for the longer ride. Cafe stops are part of cycle enjoyment, so again takes away the need for sarnies.
When I was commuting and had to carry clothing and other items, I used a Towpeak RX beam rack and slide on bag. A bit pricey, but very good quality and easy to fix on the bike of take off (simple clamp on the seat post). Would also use this for an all day ride, but for an organised sportive, not really needed as there are usually decent food stops along the way.