A lot depends on the length of the ride (both time and distance), the reason for the ride, what might be available on the route you're taking, the weather, and what sort of diabetes you have/how it's being managed.
I have type 2 and do not need insulin; however, I do take metformin so there is a possibility of going low. That said, I'm a slower rider than a lot of these folk, so my calorie and carb needs are different than theirs.
My daily 3-mile-each-way commute requires nothing more than a flat-repair kit, lights for evening riding, and water in summer. That said, I'm carrying my usual work clothes and shoes, water/food/meter/wallet/phone in my backpack, and usually either glucose tabs or some sort of energy bar, and I carry an additional layer, heavier gloves, etc. for the evening ride home.
On a training ride, when I'm riding light, I'll carry my meter, money and ID, and my phone, and wear my RoadID (which I always wear when I leave the house. I'll carry about 20 oz fluid per hour of expected activity in cool weather, and twice that in summer. My split is half water (as iced as possible) and half Ultima Replenisher (my preferred electrolyte fluid). If I expect to be out more than two hours, I'll have something like Clif ShotBloks or Jelly Belly Sport Beans in my pocket for incremental refueling; often I'll plan in a stop somewhere to refill my bottles and get something more substantial if I need it. My seat pack (which never leaves the bicycle) always has a patch kit, two spare tubes, and at least two CO2 cartridges as well as tire irons, a spoke wrench, a multitool with a few basic screwdrivers and hex wrenches, and a small adjustable wrench. I also have one of those collapses-into-nothing drawstring "backpacks" in there in case I find some incredible deal on something that I really need to jump on right then.