If you want to get rid of a propane tank, whether it's 5 gallons or 500 gallons, you have some options. You can dispose of empty gas canisters of less than 2 lbs. (such as Coleman camping canisters) in the garbage. You can dispose of larger canisters at a propane supplier. Also, giving new life to an old propane tank is definitely an option.
If all of the propane has been properly cleared out, you can convert your old tank into a meat smoker or barbecue. Once you have used all the propane in the tank, remove the main valve or the pressure relief valve. Fill the tank halfway with water and detergent to eliminate any remaining propane. You could also leave the water in the tank when cutting, or repeatedly fill the tank with water and empty it until the water runs clear. Also, you might want to burn a hickory or cherry wood fire for a few hours if you are concerned about any smell that might transfer.
An old cylinder that is used for camping lanterns, soldering torches or camp stoves (1 gallon or smaller) can be used to store compressed air for special effects like pneumatic props. You can also use larger tanks, such as 25 gallon tanks. Creating a storage tank for compressed air allows you to save money by preventing your compressor from running all the time and creating heat and unnecessary noise. Empty the propane from the tank, remove the valves and clean the tank with soap and water. You must drill and tap for a 1/4-inch thread. Repaint the interior of the tank to prevent rust around the new hole you drilled, if you wish. Use this tank between the compressor and the prop you wish to use. The compressor runs only until the tank is full and then shuts off. It will only restart when the storage tank runs low.
Many styles of yard art can be constructed from old propane tanks, including outdoor fire pits. Create a decorative look by cutting a face out of propane tank set up vertically and firmly attach legs to prevent the fire pit from tipping over. Using an empty propane tank to make a bird house is another option for yard art, and bird bath bases, compost bins, and garden tool holders are all options for empty propane tanks.