I sometimes ride downtown at night and lock my bike up, usually in sight if I can. I have a headlight, a rear flasher, a few tools, and a seat bag that I store all of that in. I don't really like leaving it all just sitting on the bike, but I don't want to take the seat bag with me as that's a bit awkward. Does anyone have a better solution for keeping the small things safe? I've considered abandoning the seatbag and just riding with a backpack/messenger bag all the time.

At the very least you can squish one of those thin plastic grocery bags into your seat bag, then you can put the seat bag & other stuff in that bag when you get off the bike. Not a very pretty solution, but easier to carry around than an awkward seat bag.

1. I have a Cateye tail light that has a lockscrew in it to secure it to the mounting bracket, which is bolted to my rear rack. That way, people cannot just pull it off the bike. They have to go to the trouble of having a screwdriver to get it off.

Here is the manual for the light. TIP: Use your own screws and nuts if mounting to a rear bike rack. Their plastic washers strip easily and come loose.
https://www.cateye.com/images/manual/ld560r_usa.pdf

2. Use a hose clamp to secure a light to a handlebar that normally would take a rubber/plastic strap. Put a strip of old bicycle tube around the handlebar surface first surface in order to protect it from getting scratched.
I did this with a small Trek light that came with a velcro strap that slid through a slot on the back of the light to hold it to a seatpost/handlebar. I used a small hoseclamp and mounted it to one of the bars on a rear rack. Worked great. You can get them in really small, narrow sizes at hardware stores.

3. Put your tools in an empty water bottle, and leave it on the bike. Put some paper toweling in there to keep them from rattling around when riding. I even used to have a plastic water bottle that was a mini-tool case. The bottle opened lengthwise, like a mummy case - exposing the tools, all arranged in their compartments. If you have a third water bottle mounting holes under the downtube on the bike, this would be an excellent place to mount it permanently. Who wants to drink from a water bottle that is held below the downtube and has all that filth from tires thrown up on it all the time, anyway?

4. I have heard of people using their seat tube for secret storage. Or in a handlebar. Though I don't know if these work well. I wouldn't use any tube that is carbon/composite for this. Metal-only, I would think.

5. Biologic, makes a seat tube stored bike pump, I think also a handlebar tool kit. https://www.thinkbiologic.com/products/postpump-seatpost


Of course, safetly first. Think about if there could be any safety issues with storing things in your seat tube or handle bar before doing it. Be careful.
This is about all I can think of right away.
Use your creativity, and you may not need any saddle bag on your bike.

Front Panniers, I have that stuff & more, in them.
Dynamo powered wired Lights stay on the bike...

Rowan and I picked up these ... the blue one is a duffle bag and the purple one is a backpack. You can see how small they are all packed down into their little bag. When they are pulled out, they are quite a decent sized duffle bag and backpack. We've been using the backpack a lot on our cycling tour for everything from putting stuff we want to secure (remove from the bicycles) to carrying groceries.



Check your outdoor shops. You should be able to find these ... then toss your seat bag, and whatever else you've got into the backpack or duffle bag (whichever is more convenient for you) and carry away.

That mini backpack looks nice. I might be able to stuff that down in my seatbag with everything else.

The water bottle tool storage idea sounds nice, except I only have one water bottle cage and I definitely like to keep that for water. As for hose clamping the headlight, I'm not sure how that would work. This is my headlight:

https://i.imgur.com/aVYIs.jpg