We have in-laws and an older friend of the family with us, so I've been researching this, also. Supposedly right at the entrance to DH, there is a community college (KCC) that has nice grounds and a cactus garden. I am hoping that they'll go walk around that area while we hike; but, honestly, most of our folks will be happy sitting, talking, and people watching. It'll be too complicated with my group to really split up.
I think you want to do DIamond Head first thing in the morning because of the heat. Also, the swap meet closes around 3, I believe, and I've read on TA that some vendors may wrap things up sooner sometimes. We are going to do do the USS Arizona as early as possible (8, I think... We can reserve tickets tomorrow!), and then we will go to the swap meet after... Some of the DE on TA thought we would be at the swap meet around 10:30-11 doing that. We will then do Diamond Head on another day when we can be there as soon as it opens. We will either go to Wakiki after, or we will do a driving tour of other areas of the island after.