If you are staying on the east coast of Kaua'i, the drive to the west coast make a nice day trip by car.  The only working sugar mill is located at Port Allen, and they offer tours.  Call ahead, since the tours are limited.  Hanapepe and Waimea are agricultural centers for sugar cane, and both have boat harbors.  Salt Pond Beach Park is just off the highway at Hanapepe - follow the signs.  A nice beach, basic facilities, and limited camping.  

The little town of Waimea is the gateway to the Waimea Canyon, which is a "must see" while you are on Kaua'i.  There are two roads to Waimea Canyon, but I would recommend the Kokee Road, which is a right turn 3.6 miles past  the town of Waimea.  The sign is easy to spot from the highway.  Just follow this very steep road with lots of curves, and stop at the lookouts along the way.  Take your time, and enjoy the views along the way.  After you leave the main highway, there are no gas stations or facilities other than a primitive bathroom at the main lookout, so take your own water and food, and fill the car with gas.

This side of Kaua'i is very dry, and if there was no irrigation, no productive crops would be possible.  Irrigation ditches were established by the Robinson family when they originally acquired this land from the Hawaiians, and these ditches are still in use today.  The ditches can be seen from the air and you can catch glimpses from the Waimea Canyon roads.

Barking Sands is a missile testing range, and military airport.  Restricted access is usually granted to civilians if you provide identification.  Major's Beach is worth visiting - the longest sand beach with the fewest people on it you will find anywhere.  If you are adventurous, drive to the end of the Kaumuali'i Highway past Barking Sands base, and drive five miles further along the coast to Polihale State Park.  (Consult a guidebook for detailed instructions.)  This is as far as you can travel by car along the west side of the Na Pali Coast.