The Volcano Discovery Centre (free entry) is located at the small square just next to the main bus terminus and Sai Kung waterfront. It is open from 9am to 5pm daily and makes for an short educational experience to complement visits to the nearby UNESCO Geopark locations accessible from Sai Kung.
Firstly, this is an exhibition hall with 6-7 different displays related to basic volcano formation and particularly that of super volcanos like what existed here in the Hong Kong region 140 million years ago. There is a nice display about how the previous existence of the super volcano was discovered as well as the outstanding geological features it left behind and which you can see in various places around the Geopark.
Particularly interesting to learn about are the rare hexagonal rock columns that can be seen in great numbers at the East Dam of the High Island Reservoir. There are also rock and volcanic material samples on display, something geology fans will appreciate.
Note: During normal times, you can arrange guided tours of the UNESCO Geopark through the Discovery Centre. However, those services have been suspended during the pandemic and will likely remain for some time. On the official website, they still provide information for the guided tours. It is worth reviewing and potentially reserving online in advance if this is something you are interested during all returns to normal someday in the future.