Thanks, Shar! The Koko head hike migh be shorter than Diamond head hike, but it's gruelling! But the reward is an amzing commanding view. I would recommend bringing water and maybe even a lunch with you to the top. I saw some people having a little picnic up there and regretted not doing the same. I took the Bus there. Easy enough to get there for 2 bucks. Go in the morning so the heat isn't to bad, and the trail isn't too crowded.

Diamond Head is easy enough to do since it's so close to Waikiki. Little bit longer but not as tough. Great view up there too.

There's also a great hike to Laie Falls on the north shore. Tiny waterfall, but you can swim in the water there. 1.5 hr hike.

The Koko Head hike is really more like a work out rather than a hike. You walk straight up old military rail-ties. It is completely exposed ie will be hot so best done early in the morning or in afternoon after 3 or 4. In January it will get dark earlier and you will want to be off the trail before sunset.
The view from the top is spectacular and there is an old bunker on which you can sit to take it all in.

Another beautiful hike nearby goes up behind Hanauma Bay and can be taken to the coast line where you may see whales in January. It is in my opinion more pleasant because you can actually take you eyes off the ground you are walking on and take in the surroundings. The Koko Head hike is really for a work out.