I really like the Wahiawa Botanical Garden, a rainforest preserve! They have bunches of orchids and bromeliads, and there's even a cinnamon tree. It is on Oahu, in the town of Wahiawa, about a 45 min drive from downtown Honolulu. Best to go on a Sat or Sun if possible because otherwise you have no staff to guide you thru the garden. But I took the "self guided tour" the first time I went and i loved it! Bring mosquito repellent- it is a rainforest. :) Cutter's works, OFF doesn't.

The above recommendation is good. I prefer the following:
Sadie Seymour Botanical Gardens Kona Outdoor circle of Kona Educational center 329-7286 76-6280 Kuakini hwy
( Good collection of plants from Hawaii and around the world and reasonable identification of them)

World Botanical Gardens at Hakalau about 5 miles north of Hilo Hawaii

Amey Greenwell ethnobotanical garden (Bishop Museum) at Capt Cook about 8 miles south of Kona

For more information, call the Garden at 808-323-3318, email to agg@bishopmuseum.org, or drop by the Garden to see our plants and talk to staff about your needs.

Queen's Garden on Kuakini St between Nuuanu and Liliha Sts

Foster Garden much like the Sadie Seymour garden in Kona - Admission fee

On Oahu, Hoomaluhia Botanical Garden (open 9-4 daily) has plants from tropical regions of the world. GUIDED NATURE HIKES are offered at 10:00 a.m. on Saturdays and 1:00 p.m. on Sundays. Please call (808)233-7323 to register.

Also on Oahu, Lyon Arboretum (open 9-4 M-F). Free guided tours: 10 to 11:30 a.m. Mondays to Fridays, limited to 12. Weekend tours are scheduled to resume in the fall. Call (808)988-0461.