Any good ideas what to do with old propane bottle?

NEVER - Never attempt to cut a propane tank, UNLESS you are an experienced welder. There are special safety precautions that must be followed to avoid an explosion. We just had a friend who lost their son because he attempted to use a cutting torch on an "Empty" tank. Take it to Walmart and exchange it. They take out of date tanks, which are then tested and have new vales installed. If you want an air tank, go down to Walmart, Harbor Freight or Northern tools and buy one.