This one is very special, and I strongly recommand it in Maui. It is so HUGE, just phenomenon. You won't be regret if you see it. Trust me.
By the way, it is very cold on the top even in July. It is above 10K feet high.
The active volcano in Hawaii Island is also a must. There was one time I stood 3 meters away from the red-hot lava that just coming out -- certainly it is safe, the lava moved very slow and the rangers were in site. You may not see any active volcano so closely anywhere else.