Cham Island Hoi An Vietnam

If you're pressed for time and need to choose between My Son and Cham Island — no contest, go to Cham Island. 
That said, generally the tours of Cham Island are over-hyped. They talk about visiting pagodas and this and that, but here's the real deal: most of the island is controlled by the army and is still off limits. There's one part of the beach with a pier and a restaurant where all the tours dock to feed their customers, and then there is the village a short boat ride down the coast. You can't go into the hills or venture any further along the beach without incurring the wrath of the military. 
If you're just after a basic tour, including transport, some beach time, snorkelling, a walk through some greenery and a visit to a village or two, then any agency in town will be able to help: expect to pay around US$25 for the fast boat and US$30 for the slow boat. 
But the primary reason to go is for the coral reefs along the coast, so if this is what you're after it's worth looking at one of the  companies that specialises in snorkelling and scuba diving: Rainbow Divers, Blue Coral Diving and Cham Island Diving Centre all receive consistently positive feedback. 
The coral is pretty good, (by Vietnam standards), but, though it may cut into their business to say this, it's all easily accessible by snorkelling, so don't spring for the diving if all you want to do is peek at the sea creatures. However, if you're a certified diver looking for action, all three operators have a wide variety of dives on offer — including drift dives and night dives. 
From early November to February the island is inaccessible due to high seas — and the weather starts getting dodgy in October so plan accordingly. During fair months, however, you have the option of a daytrip or an overnight stay in tents on the beach with a big bonfire at night