before you can go to the inside of Sin, You will gain control of the airship. There are several secret destination you can go to.

There is a seach function wich enables you to find additional locations on the map. The X and Y coordinates is what matters here:

X-axis Y-axis Destination
11-16 57-63 Baai temple (Anima)
12-16 41-45 Sanubia Sands (Valley of the Cactuars)
29-32 73-76 Besaid Falls (Waterfall Way)
33-36 55-60 Mi'ihen Highroad (the ruins in the water)
39-43 56-60 Battle site (Mushroom Rock Beach)
69-75 33-38 Omega Ruins

Good Luck