My guess is also sugarcane.

They really ought to be able to do geothermal - especially on the big island.

They do have a geothermal plant - on the Big Island.  They originally had grand plans.  They were going to lay underwater cable to Oahu, where most of the population is, and export geothermal energy from the Big Island.  

But it didn't work out as planned.  The plant works, but is more expensive, more difficult to maintain, and more polluting than they imagined.  Many Native Hawaiians object to the plant on religious grounds.  Plans for more geothermal plants were shelved, as was the plan to export energy to Oahu.

Though maybe they'll dust them off again, if Lingle's serious about this.