I was recently asked to suggest a “checklist” of items one should carry for a brevet. Here is an old list that I re-use from time to time, with some comments below.
The following list is an amalgamation of two lists I used to use - one for brevets and one for touring - long brevets (300k) and short tours have fairly similar requirements.
Depending on your preference, this could include a handlebar bag, top-tube bag, frame bags, or even panniers. (Yes, I have ridden a brevet with a pannier once). A good handlebar bag is great for brevets when you might need quick access to your cue sheet, brevet card, phone or other items without having to root around too much. A Spare tube / puncture repair kit can sit in a saddle bag. With experience, you will evolve a system and not need to remember what is stowed where.
Keep at least one filled up at the start, and plan water refills based on the route. Some riders prefer to convenience of hydration packs, but I have never needed one.
Good lighting is very important for rides that happen outside daylight hours. For 200k and longer brevets, the organiser will most likely mandate front and rear lights (must be white and red respectively) even if the event begins and ends in daylight. Lights can be classified into those to see with (cast light for you to see the road) vs those to be seen (i.e. make you visible to others).
- Front lights:
Battery powered front lights have come a long way, personally I really prefer the beam shape of the German syle lights like the Ixon IQ - these are the best to see the road ahead. Front lights that look like flashlights are only good to “be seen” - most don’t cast enough light on the road. Even if you have dynamo lights, keep at least one spare white light like a head-band light which can be useful if you even have to fix a flat in the dark.
- Rear lights
Brevet rules mandate at least one steady (i.e. non flashing) rear red light. I always prefer to carry two, one attached to the seatpost and one on the rear rack / fender.
- Front lights:
Fairly self explanatory. Before the days of GPS a simple wired bike computer was enough; today even an old phone is a good enough GPS. Personally I recommend the Sigma BC 16.12 series computers.
- Rider number placard
Brevet rules require the rider number affixed to the bike. An easy way to do this is to print your rider number on paper then get it laminated, punch holes in it and ziptie it to the bike. You can also add your name and emergency info to this.
Tools / Spares
- Puncture Repair Kit:
You will need, at a minimum:
- Tire Irons
- Patch Kit
- One or more spare tubes
- Glass marking pencil
- Additional tools include a multi-tool or:
- Allen keys
- Flashlight for emergency repairs
- Zipties: carry a few, never know when one might be useful
- Rope A few meters of cotton rope is always good to have to tie down stuff in an emergency
- Fiber fix Spoke May be necessary on very remote / long rides
Clothing / Safety
- Helmet (normally mandatory) and gloves
- Reflective Vest (mandatory) You can get one that is basically like a sash - not a full vest
- Change of clothes for longer rides
- Trouser clips / reflective Ankle bands 
- Cash and Cards
- Charged cellphone and possibly a spare phone.
- GPS device / smartwatch
- Maps and Cue sheets
- Rain protection: rain jacket, rain legs, shoe covers
- Bike Lock & key
Food & Medical
- Chocolate: Lots of sugar and good taste can really elevate your mood. remember that on a hot ride it can get soggy :( Energy bars are ok, depends on personal taste.
- Electrolytes: depending on your preference, carry some Electral / Enerzal or equivalent.
- Salty / Fatty snacks: I prefer roasted groundnuts - good amount of energy, and tastes good, readily available.
- Tissue / wetwipes
- Hand sanitizer
- Small hand towel
List of things to check the evening before the ride:
- Tire pressures
- Recharge all batteries including spare batteries (lights, GPS, headphones/bluetooth etc)
- Upload / Save route data into phone or GPS
- Debit cards if needed for ATM Controls, or other apps needed for location vertification
- Get some rest!