Starting around mile marker 6 or 7 of the Road to Hana, you’ll pass groves of rainbow eucalyptus trees fairly often (depending on where they are in their shed cycle sometimes they’re easier to spot than others). Parking is pretty tricky along this stretch so unless you’re passing through early sometimes it’s not so easy to pull over. 

But the Keanae Arboretum has a grove of them with some parking across the street.