Fixing this took maybe 3 minutes- let's learn a new thing!
Step 1: Hey what happened?
As you've definitely already realized, i'm posting this in October and live in Rhode Island- so why do i care about this right now? Because flip flops are great indoor winterwear because they keep the cold floor separate from the feet without having to be trapped inside slippers or socks. In fact, i found i stay warmer indoors when i wear a hood or hat, sweater, pants, and flip flops. Something about being able to feel the cold air.
Anyway, drill a hole across the end of the peg, above where the stopper that broke off used to be. By peg i mean the thing that goes between your toes.