Can't tell you much about Java, but if you are already in Danum Valley, have you thought about checking out islands off Semporna (especially Sipadan) if diving is you focus? There are no luxury resorts near Semporna that I know of but decent enough. Certainly nothing close to St. Regis Langkawi or Amanjiwo standard tho. August isn't the best time for Langkawi (it's the southwest monsoon then and the sea can get a bit rough with some rain).

There should be a (JW?) Marriott opening at Perhentian Islands this year although they haven't announced the exact dates yet. I've seen it under construction for the past few years and it seems like a wonderful property. Diving in Perhentian is certainly better than Langkawi. And Semporna/Lankayan Islands near Sandakan takes the prize.

If you like wildlife, Sabah Borneo is the place to be, both on land and under the water. I haven't been to Danum Valley but I've heard wonderful things about it (the town just outside of Danum Valley, Lahad Datu, is supposedly terrible, so is Semporna just before you board the speedboat to the islands). I've only been to Kinabatangan twice and think it's a great place to spot wildlife.