Diamond head has a nice view and is an incredibly easy hike for the whole family, but if you only have time for one hike on Oahu there are other easy hikes that are only slightly hard to access. The view of the ocean, Waikiki and Honolulu is nice, but the crowds are getting worse, and most Diamond Head Hikes include waiting in line behind hundreds of slow tourists. If you do stroll up Diamond head I recommend an early start, or bring a flashlight and watch the sun plunge into the pacific from the top (high green flash potential here!).
"After the first glass, you see things as you wish they were. After the second, you see things as they are not. Finally, you see things as they really are, which is the most horrible thing in the world."
--Oscar Wilde on Absinthe