Flip-flops under the chair legs work fine.

Yes. Just slide your flip-flops (or sandals, or a small piece of cardboard) under the chair’s back legs where you sit. It distributes your weight and stops the legs from plunging straight down.

The problem is basic physics: small round feet sink fast in soft sand because all your weight is on a tiny contact patch. Adding a flat surface the size of a flip–flop spreads that force out. It’s the same reason purpose-built “beach chairs” have wide plastic feet or strap-on sand anchors—but you don’t need new gear.

If you’re in really loose, dry sand (like the top of a dune), you might need a little board or a folded towel under all four legs. But for normal beach sand, just the flip-flops under the back legs (where most weight goes) is enough.

It’s not elegant, but it beats digging yourself out every ten minutes.