The Island of Kauai is not particularly suited to biking, unless you have a mountain bike and are willing to use the backroads. No one is going to give you a map, but there are number of very interesting routes and many points of interest along the way.   This is the best one in the Koloa area. Start near Poipu and follow Poipu Road east to the end. Continue along the dirt road towards Maha'upelu. As you approach the sand dunes, look for a track going south. This will take you to Makauwahi Sinkhole (link).   The view from above is great, but in order to get into the caverns, you have to crawl through a cave entrance around the side.   There is often archaeological work here.  Try to get into the cavern if you can. 

After looking at the sinkhole, follow the road out to Maha'ulepu Beach.   This is our favorite beach on the planet.   To swim, go right over to Gillins Beach.   To tour the lithified dunes and headlands, go left.   I was able to ride the tracks out to the last bay below the mountain.

Beyond Haupa Bay, horse trails climb over the ragged lava flows towards the Hoary Head Range, a 2000 foot impenenetrable line of dormant volcanoes, a wall separating the south shore from the rest of Kauai.   The rough a'a can shred bike tires, but I was able to bike up the hill to a viewpoint.   Anywhere beyond is so rough and difficult that there are no trails over the range.   Returning back to the Mahaulepu Road, I turned off onto another side road that heads over to the now-abandoned Koloa Mill.   Along the way, cane toads sometimes can be seen at the side of the road.   White egrets comb the former cane fields for insects and worms and tropical birds sing in the foliage along the red-dirt road.  On my first two trips to Kauai, the fields all around were planted with sugar cane and the mill was in full operation, but the last mill closed about 15 years ago.   Farther up the island we saw an old mill being torn down.   The Koloa Mill is still standing in the basin between Mount Haupa and Poipu.

From the mill a road runs north to the Hoary Head Range.   On fork goes to the Waita Reservoir.   I rode my bike up there and was pleased to find a flock of nene, a type of goose and the state bird, now endangered.   They have a clumsy way of walking and they scrambled away into the reservoir at the sight of my mountain bike.   At the end of the track I also ran into a feral boar.   We watched each other for a while before he worked its way into the bush.   On this trip to Kauai I saw 3 boars, but also saw a lot of boar damage off the sides of trails.

The other fork runs straight at the mountain, then goes through a tunnel to the other side.   Some years earlier while the mill was still running and sugar cane fields were still active, I took it upon myself to cycle through the tunnel.   The surface is dirt, it runs straight, is unlighted, and is a half mile long.   I had hoped that no trucks would be coming through because it is only one lane wide.   No such luck.   Halfway through the tunnel a pickup track came racing through.   I pulled my bike into the ditch and pressed myself against the wall.   The truck driver saw me glued to the rock wall and slowed down.     I went through the tunnel to see the valley of Kipu on the other side before returning back through.

Coming back to the Mill, I worked my way over to sections of the Koloa Heritage Trail and then back to Poipu. Along the way, 5 extinct volcano cones can be seen, the last ones to erupt on Kauai.   All in, the ride is about 25 km, but with beaches, headlands, sand dunes. sinkholes, caverns, lava flows, wildlife, reservoirs, tunnels, volcanoes, and historic mills, there is much to see. I have done this route a few times and will return to Kauai to do it more times (if I am lucky).