The ruins of the Old Sugar Mill of Koloa stand at the north edge of town as a compact but resonant memorial to Hawaii's plantation era. What remains is the stone chimney stack and the shell of the mill building, preserved in place since the mill closed in 1996. Bronze statues of the ethnic groups who worked the plantation — Japanese, Chinese, Filipino, Korean, Portuguese, and Hawaiian workers — stand nearby, one of the more honest acknowledgements of the plantation labour system that you'll find anywhere in the state.
The site is free to visit and takes about 20 minutes. A short interpretive trail explains the sugar-making process and the mill's history. It's worth combining with a look at Koloa town itself, which retains several original plantation-era buildings along its main street — small wooden storefronts that predate the resort development of Poipū by several decades.