I am not sure why you necessarily need to go monk for AC. Goliath druids, unlike every other kind of druid, has absolutely no problems with armor whatsoever.
When you turn into a giant (a kind of humanoid), your equipment scales up with you. So you can use whatever armor and weapons you want.
So mostly, I do not see too much reason to dip monk. WIS to AC is nice, but without scaling on that AC boost, it will eventually get outclassed by normal armor. And while the unchained flurry is slightly better than regular flurry for multiclassing...it still feels rather 'meh' for the loss of spell levels, delay in wildshape (I would not even dream of multiclassing a goliath until after I get regeneration at level 12 myself) and a loss of BAB a lot of the time.
I would just use either the racial proficiency for battleaxe/warhammer (they hit like greatswords when large) or a scimitar (18-20, obviously) and be done with it. Combat reflexes too since giants have great reach.