What to do with old tires?
I usually gift them to people who need them.

Edit: To expand more on my answer, I use rather expensive tyres and I want them in mint condition. So after a couple of months I'll most probably change them. The old tyres can still give 6-12 months of quality riding time to someone. So I'll post on local riding forums with closeup photos of the tread and usually people will get them.