Atlas - Calm Belt
Judging from the maps shown so far it appears that there are islands within the Calm Belt however not a single one of them has been mentioned so far. However since the Straw Hat Pirates went into the Calm Belt for a short period it counts as a place they've been to.
Places already visited by a Straw Hat Pirate:
Places not yet visited by a Straw Hat Pirate:
None been mentioned so far. It that should change at some point, it'll be mentioned here.