Place them where you'd like a garden. Fill them with good black dirt and plant flowers or vegetables. If the tires are big enough you can use them for sand boxes. Here's a bunch more of great ideas!
Hi Em: Well, you can make a bunch of things :) Even a swing if you have a big tree! Oh, I remember my tire swing - sigh :/ Anyway, back to you:) Here are some sites with great ideas:
or, you could make a shelter for feral cats. There is a piece of plywood, painted to resist rain and snow, placed between the tires to give a floor for one, and then another on top to make a roof. You can screw an 'awning' above each door that you cut out, about 5 or 6 inches wide and maybe about 5 or 6 inches high. Put straw inside for warmth. You can contact some cat rescues to see if they need any tires to make shelters, too. To view the teeny tiny picture at the bottom, right click on it and then select view image. Good luck
One word of caution...old tires retain rain water and it becomes a breeding ground for mosquitoes.