Some people recommend throwing them in your regular garbage but that could be dangerous just in case it decides to explode inside the trash truck due to the propane vapor left inside the canister.
I would check with your county or city and see if they have a hazmat disposal option as some places will take propane tanks including the small mini single use propane tanks for recycling.
If you have a landfill nearby where you can drive too they may have a spot designated for hazmat waste where you can drop off the mini propane tanks.
I personally make sure they're drained fully and then use a chop saw to cut them open and then my scrap yard takes them once cut open.
The mini propane tanks are made from metal so they can be sold as scrap metal once cut open.