That's a great question because my feet hurt a lot especially when I do yoga. I stretch them and wear orthodics.

If you're interested in feet-strengthening cardio, Ellen Barrett teaches low-impact barefoot classes with that express purpose. You might enjoy her "Barefoot Walk" video in this collection:
https://grokker.com/fitness/collection/cardio-walk-with-barrett-and-fonda/603ad1c4b356ab7d31b48e73