Those with a special interest in tropical gardens will find the reservations only Tuesday through Saturday walking tours worth reserving well ahead of time, and especially so on weekends. 

If possible, pick a day with wind and slather on the mosquito juice -- long sleeve pants and a long sleeve shirt in khaki or other dull colors plus decent hiking boots help keep the bugs off. Windy days also maximize the "hose the tourists" action at the Spouting Horn. 

SHOPPING TIP: Vendors around the Spouting Horn sell a lot of junk, but also some of the best Ni"ihau shell necklaces in the islands. Just make sure to get a certificate of authenticity and an address so your wonderful necklace made from tiny shells can be repaired or supplemented with ear rings and such later.