I am only on my second set of tires. That means I have used up 4 tires thus far. One became a tire swing and the other three I have made into hose pots. Hose pots are something that its hard to find second hand but they are great to make from old tires. It also side steps the concerns about planting in tires and chemical leaching.
Here is the best page with ideas for car tires. Some are really elaborate.