Cham Islands, approximate 20km away from Hoi An and the mainland, is a group of 8 islands. However, only the largest one, Hon Lao, is inhabit.
The islands offer some beautiful beaches that are not crowded, some nice dive sites nearby, a little fishing village where you can observe local life and some nice walks. So it is certainly a place worthwhile to consider if you need time to relax and escape the busy town of Hoi An.
The island itself has not a strong tourist infrastructure with limited accommodation. Most visitors either go here with an organised day trip (mainly Asian tourists) or as part of snorkel / dive trip. it is also possible to go here by your own, requiring some planning and getting a permit.
However, the island is also a military outpost, so some parts of the island, especially to the east, is off-limit.
For a nice beach experience, Cham Island is a nice option for a day-trip or an overnight stay.
My stay here is covered in the blog entry for Cham Islands.