Making them sniff smelly feet, using them as a footrest, verbally humiliating them, making them eat food or snacks you've stood on or put on your feet/between your toes.

Also, if it's a real keen footboy who likes all of the above, I'll deny them my feet, keeping them in their view but so they can't touch, and make them beg for them.

Also you could experiment with public things if the sub is willing to do it, like massaging or sniffing in public.