Just makai beyond mile marker 18 on the Hana Highway is the small road leading down to Wailua Town. While there isn't much for visitors to do or see in this taro-growing town, the spot is worth a mention due to the fact it's the only place you can catch a glimpse of Lower Waikani Falls. This falls is on the Wailuanui stream and is one of the largest falls under the highway. From the town road, you should be able to get a partial view of the falls by looking inland, back up the mountain. It's a nice side-trip if you have the time. If not, you can get a more intimate view of another falls on the same stream in a few minutes. Upper Waikani Falls is just ahead.
Wailua State Wayside Park is just before Mile Marker #19 on the mountain side of the road. Take the stairs on the right for a fantastic view of Wailua and the taro fields below.