People with respiratory or heart conditions should consult with their physicians before travelling to Haleakala due to its high altitudes. If you want to make an Oahu cab driver giggle, tell one you're staying at the Outrigger near the ABC store. Not all of the Waikiki hotels are on the beach, so make certain to get what you expect from your accommodations. In Waikiki you can find an ABC store just a few stores down from another or even one on each corner at an intersection. Because of their frequency they don't make for great landmarks for directions but they are very convenient for sundries, beach supplies, or small grocery items.
Prices are slightly higher than grocery stores.
If you want to go shell collecting in Hawaii you'll probably be disappointed. The shallow reef flats lead to a steep drop off about mile out to sea and the tide won't push the shells to the beach. If you happen to be a really good beachcomber maybe you'll be able to find puka shells. They are the last remaining bits of a Cone Snail shell, which is cone-shaped and closed at the big end. After the cones are tossed in the surf and coral rubble the pointy end is broken into sand.