You might encounter tunnels. Some of them are without lights, which is why I recommend at least a red tail light. Something small and fiercely flashing.
I also recommend bringing along a bike lock. You can either choose a small one that prevents a grab-and-ride theft of your bike. Such thievery is more common than you might think.
The other option is, of course, a proper heavy bike lock that withstands simple tools for stealing your bike. If you bike in a group, you can lock two bikes together with just one heavy lock.
My current choice is to just bring the small one. They are strong enough to stop thieves during a lunch break, since you are likely to be close enough to notice and stop any tampering with your bike. At night you can often bring your bike to your room, or put it in the garage of the hotel where it is their responsibility to keep it safe. But, if someone steals your bike, your vacation is just about over. So think twice about locks.
You also need a water bottle, perhaps even two. How much do you drink while biking, and how hot is it going to be? I stick to plain water, but it is up to you.
Finally, bike repair. I am not much of a bike mechanic, but you absolutely need the tools and skills to replace punctured tubes and tires. On all my trips I’ve brought one extra tire and two extra tubes. If you end up replacing a tube, then you still have an extra tube and tire in case you realize the cause of the puncture was something sharp still stuck into the tire. Then buy new stuff at the next opportunity. On average I get a flat tire for every week of biking. In Argentina I once had both tubes punctured at the same time.
It is also a good idea to learn how to adjust your gears. It is well worth the time and money to pay a local bike mechanic for a tutorial. It is also a good idea to know how to replace brake pads (unless you have disc brakes). If you service your bike before you go, and your trip is just a few weeks, then you wont need extra pads during your trip, but for longer trips you don’t want to wear out your brake pads without replacements.
You need a bike multi tool to tweak bolts and screws, but pick a small one since they’re quite heavy. I have one which has a special tool for fixing broken chains, but I have never used it. It is actually one of the only things that worries me, being stuck on a remote mountain with a broken chain.
I have also traveled with a small wrench/spanner just to wrest off the pedals from the crank arm when packing the bike for going home. I really don’t think you need it. You can just as well go to a local bike shop and ask for a little help.
Bike gear is with a few exceptions often available within 100 km or so from wherever you are, so don’t worry too much if you realize you need another water bottle.