In S1e4a "Deep Sea Johnny" Susan and Mary are wearing swimsuits and sandals. (Note: This is the first time their feet are shown)
In S2e8b "Johnny the Kid" they wear swimsuits and sandals.
In S2e9b "Johnny Meets the Pork-Ness Monster" has Susan and Mary wearing sandals for the first half of the episode.
In S2e10a "Johnny X Strikes Back (JX4)" Susan has one shoe on and one shoe off.
In S5e4b "Johnny of the Deep" Susan is wearing a tropical clothing with sandals when Bling Bling Boy used a ring to make Susan fall for him.
In "Johnny vs. The Tickler", they are having their bare feet tickled by "The Tickler", Wacko's evil brother.
In S5e22b "It's Du-Kay, Johnny" the twins just woke and realize that one pair of their "GIL" slipper is gone (Mary lost her "G" and Susan lost her "IL")
Near the end of S6e15a "Johnny Unplugged" Johnny uses Susan and Mary's stinky feet to get rid of mountain lions.