Koloa is where sugar came to Hawaii. The old mill town on Kauai's sunny south shore was the site of Hawaii's first commercial sugar plantation, founded in 1835, and the remnants of that history — a stone chimney stack, a bronze plaque, a row of monkeypod trees — still anchor the little main street. Today Old Koloa Town is a compact strip of wooden storefronts selling shave ice, surf gear, and Kauai-grown coffee, pleasant enough for an hour's wander but not the reason you are here.
The reason you are here is Poipu, five minutes south, where the best swimming beaches on Kauai line a coast that gets more sun than anywhere else on the island. Koloa is the gateway: quieter, cheaper, and close enough that you can walk to Poipu Beach Park in the morning and be back for a plate lunch by noon. The surrounding area — Spouting Horn blowhole, the National Tropical Botanical Garden at Lawai, the red-dirt trails above Koloa — gives you a few days of exploring without needing to drive the whole island.