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.
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.
, makes a seat tube stored bike pump, I think also a handlebar tool kit. http://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.