Cotton or linen works best.

A plain cotton or linen blanket stays cooler than synthetic fleece or polyester.

Those cheap “sand-proof” microfiber blankets trap heat because they don’t breathe. Cotton and linen let air through, so the fabric stays closer to ambient temperature instead of baking in the sun. A lightweight Turkish towel (often called a pestemal) also works—it’s thin, dries fast, and doesn’t get crispy.

If you want extra cooling, look for a blanket with a reflective aluminized backing, like the kind sold for camping. That reflects sun away rather than absorbing it.

Honestly though, you can just use a bath towel folded over. It’s not fancy, but it works.