There are so many things other than the bike that are pretty much indispensable, especially tools, but the thing that comes to mind is my dynamo light with USB charging.
Most things I could work around but not the dynamo set-up. It's not just that it's dynamo but also how good the lights are. The front one is a game-changer for night-time cycling on rural lanes - much better than the battery pack light I had before it - and the battery pack light cost more (than the dynamo front light and the dynamo wheel together).
The dynamo-powered USB charger means that on cycle-camping trips, I can keep all my electronics working without worrying or planning charging opportunities, as long as I'm riding in daylight when the dynamo isn't needed for the lights.