I have a long commute so I feel like I bring lots of stuff. I think that commuter bike design will be very different for short vs long distance commutes.

I bring a small assortment of bike tools (mainly patches, spare tube, tire levers, multi tool allen wrench thing, mini pump, bike lock), bike computer, camelbak for water, food for the ride (I get so hungry when I ride), things to clean up myself at work, work clothes, folders with whatever work documents I need to carry back and forth, lunch for work, rain jacket, lights, and I think that's it.

It feels like a lot of stuff, but I suppose I do use it all. I could pack better and maybe condense to a trunk bag, but then it feels stuffed so I prefer to use 2 rear pannier so I can be sure to easily and quickly pack stuff and still have a little room for some of the random farmer stands that I am starting to find on my route home.