I grew up calling flip flops “thongs” and think it is still legitimate to do so today. Yes, sharing a name with a piece of underwear is a bit embarrassing, but consider the alternatives.
“Flip flop” automatically implies it is only worn at the beach, is made of foam, and can be purchased for $3 at Old Navy. This is not true of thongs. They can be made of leather and have great sole support systems.
“Sandal” is also an inappropriate term. Sandals usually have multiple straps, sometimes a heel for women, and are a much larger umbrella term for summer footwear. Thong is more specific.
(btw, I am out of town Fri-Sun and will be comatose on Monday. so, no posting.)