Thanks for all the great tips, gang. My current loadout is...
Definitely Going to Bring List
Kiboko 22L (carry-on, will hold everything, fits under seat)
Smaller day-bag for hikes/walkaround
Wide scenery lens rental -- Zeiss 21 or Canon 16-35
24-70 F/4 IS + hood
Tripod / Ball head / mounting plate / shutter release
Lens filters -- UV and CPL
Lee filter kit + ND grads + Big Stopper
'Photo trip stuff' -- air blower, cleaning kit, filter wrenches, etc.
Batteries / charger / backup cards / yadda yadda
Flashlight (for pre-dawn shooting)
Something for drying/cleaning gear from saltwater splashes (tap water bottle and a facecloth)
The Maybe List
One small prime (28, 40 or 50)
70-200 F/2.8 IS II + hood + tripod collar
2X III Teleconverter
Waterproof camera for snorkeling
Trash bags or a storm cover
The Not Bringing List
100L Macro (again, the 24-70 has that excellent 0.7x macro option)
600 EX-RT Flash (I generally am a natural light shooter unless it's an indoor event)
I've downgraded the stellar 70-200 and the 2x to the maybe list. I know I'll get great shots with it, but it's heavy, takes up a lot of bag space and is quite conspicuous. Principally, though, the majority of the heavy photography downtime without my gf will be on a scenery photo tour where wide shots will be the focus -- the guide even recommended the Nikon 14-24 or Canon 16-35 for where he's taking us.
But my mind is not made up. I appreciate everyone's guidance on this, and I am always game for cool locations to check out!