There are two known entrances to the Sea which were both used by heroes. There are the Clashing Rocks. They roll past each other so quickly it's nearly impossible to get through. The second is the straight of Scylla and Charybdis. You cannot go around them because they will just appear in your way once again.
The island is rather beautiful and seems like a tropical paradise with a five star resort on it. But appearances can be quite deceiving. Females are in no real danger and will be treated quite well by Circe and her staff. Circe has a strong hatred of men. The only improvement is that she doesn't turn men into pigs any more, but guinea pigs because they are much easier to clean.
This Island perhaps fits the best description of a monster's island. The Sirens' Island is barren, dank and littered with wrecks and bones of the victims of the sirens. Despite this, it is considered a profound experience to hear the song of the Sirens because it can tell you things about yourself that even you didn't know and you can become a lot wiser if you survive the experience.
By far the most beautiful island in the Sea of Monsters, though this is thanks to the work of the Golden Fleece which has enhanced the beauty of the plants and animals of the Island there. For example, the sheep are the size of hippopotamus. This is the island the Cyclops (and son of Poseidon) Polyphemus lives on.
Hephaestus has a forge in the Sea of Monsters in the form of a volcano.