You could always put a few in the back and use them there for the tater tower! That way, when he wonders where they are you can say, "I'm using them out back, and they're great!
The only trouble with the tires is that some of them still have rims in 'em.
That and that the landlord said something about planting stuff in them himself...I don't know if he quite gets it yet that I really *want* to do gardening and all myself.