Be careful about using old tires in areas where you spend a lot of time or eat produce grown inside them because there is some link to cancer/carcinogen. You can run a Google search and make up your mind about that. Personally, I wouldn't risk it with vegetable/fruit crops, but I'd consider doing it with my ornamentals. If you are in the US, you can take them in for recycling since most landfills won't accept them due to it being illegal in most states. They require special processing and can be turned into other products. There's also all the crazy stuff that Earthship has done with tires, but you'd need quite a few of them. You can build houses, sheds (as RMS suggested), walls, drainage diversions, foundations for patios, lots of stuff. And then there are the "playground" applications like RMS mentioned. You can bury them half way into the ground for a mini hurdle track/obstacle course or hang them from a tree for a swing. You can cut them up, place them around in a clover type pattern, and fill with sand for a sandbox. I've also seen these secured into the beds of rivers and ponds for the fish to use as hatcheries, but I'm pretty sure that's a bad idea. Bottom line: Check with the authorities and see what you can legally do. You might not be allowed to use them outside because of pollution to local water systems, and your only option may be a recycling center.