Can anyone recommend (you've done it) any short (3 or less miles), very scenic hikes on Oahu with mountain/valley/water views?

We are going to hike Diamond Head and hopefully Manoa Falls Trail in 39 more days.

You could hike up to the Makapu lighthouse (or near to it). It's fairly short, and has pretty views.

Makapu Lighthouse is great, can't beat the views! A fun, very short, walk is the old Pali road. From the Pali lookout, it curves downhill to the right. You follow the old highway downhill, then go back up when you're ready. There are loads of pretty trails up in the Mt. Tantalus area behind Waikiki.

I second the recommendation for a Tantalus hike!