Cell phone. If no camera in phone, bring a camera. Post photos here.
Know when sunset is and bring lights with enough run time, and a little extra just in case of a flat etc. in the dark. If appropriate, bring extra batteries, and an extra light.
Check the weather report the morning of ride day. The closer to the ride the more accurate it will be.
Plenty of cash is handy.
If you think you will be exhausted at the end, being on familiar roads is easier, especially in the dark.
If finish time is important add extra time beyond just the riding time. On a century an extra hour shows up very easily. Even two.
If your bike is now working OK, don't make any changes or adjustments just before the ride. Don't depend on anything new that is not tested yet.
Cheap fruit drinks at the store does wonders for a bonk. It can get you going in 5-10 minutes. Then buy another to bring with you.
Dog cookies are a big help with an aggressive dog.
Try getting some things ready the night before. Maybe even check the tire pressure then, if the tires hold air well. If you need an extra jacket etc. do some packing the night before.
I have a special place in my kitchen that I put things like water bottles, food, etc. that I check when I leave.