Except for the more touristy Lantau Islands, the rest of the outlying islands of Hong Kong are not as well-known or frequented. They’re worth noting for the stark contrast to the city but that’s something that can be said about most outlying islands off the beaten track in any part of the world.
I’ve previously been to Cheung Chau, a more developed island that’s starting to gain a bit of popularity among those looking for a slower pace of life and it’s really not advisable to visit during the hot summers when the lack of transportation AND the heat team up to kill any spirit of adventure you can muster. The same can be said of Peng Chau (not to be confused with Ping Chau).
Most ferries to the islands depart from a terminal in Central and you can get a ticket on the spot with the same ticket you’d use for the MTR trains. A lot of the houses on the island are in various forms of dilapidation so it’s well worth a day trip to explore and walk around.