Hiking Diamond head is a must, but you don't need GPS.......
There is Makapu point to the west, but you have to be careful, if you see pineapple fields, there could be pot fields growing among them,,,,
North shore head over to Waimea and enter the park with the waterfall most know for the show LOST, I swam in there...
you won't need GPS, until you go to the BIG ISLAND...
Devastation trail, in Volcano's national park is a must.
Kiluea Caldera hike inside if not active is a must, I was in there 2 weeks before activity......
Drive to the top of Mauna Kea....walk slowly

drive carefully....
There are off road trails all around that mountian where you definately need GPS....
Take saddle road if you go from Kona to Hilo,,,,,,watch out for blackhawk choppers, and tanks on road.....all military proving grounds in there....
Nice cave called Haumana or something like that,,,,,1 mile one way, I did it alone, wife was scared,,,,,it was a dead end, so had to back track out....
Back to Oahu......
BLOWHOLE, gotta see it...
Rabbit Island, boat out to it...
Hanama bay snorkle it...
Elvis's little private beach where he filmed Blue Hawaii I think it was...
KOKO head mountain on east side near the Honolulu zoo....or sea world type place, this was years ago...
And by all means, go to Pearl Harbor,,,,,,,,
I was just being your personal GPS....but yes Garmin, can't go wrong.....go with one with the maps obviously including Hawaii...