If you stop to see the Thurston Lava Tube like most tourists do, be sure to cross the road and explore the Crater Rim Trail.  Spend ten minutes walking along the crater rim of Kilauea Iki - it is fascinating, and you will get some great shots of this smooth-floored crater (1958 was the last event - quite recent).

I decide to hike westward along the Crater Rim Trail, and then circle left (counterclockwise) down the crater wall to the start of the Kilauea Iki trail. There are an incredible number of birds singing as I walk through the tropical crater rim on the first leg of this hike. Kilauea Iki Forest Walk tropical bird sounds (Real Audio, 30 seconds, 77k file size)

The walk across the bottom of Kilauea Iki's crater bottom is a real eye opener, since there is so much plant life on the bottom growing in the many fissures.

Then I climb the steep trail back up the crater wall to the Thurston parking lot. I was puffing by the time I get to the top. 

Total distance is about 3 miles, total elapsed time was 2 hours.

A great hike!

If you decide to take this hike, take a bottle of water for each person, as well as some high energy food such as cereal/granola/chocolate bars.  Drink a third of your water before you finish descending to the crater floor.  Drink the next third of your water while you are on the crater floor, and drink the last third of your water while ascending to the parking lot.  Eat your energy bar while you hike across the crater floor - you will need considerable energy to climb back up the trail to the parking lot.

 Start out from the parking lot no later than 3 hours before sunset. You don't want to finish this hike in the dark.