You will have a fabulous time on this trip. I did it last year including the HK add on and the trip to Lantau.
As DavidG says, it is a full day, but you see so much.
On the day I went we travelled by Coach and boat to the Island. Visited the fishing village and a trip on another small boat. Then to the Buddha and Monastery, followed by lunch. Then a trip by cable car back to HK
In the evening we did a boat tour through the harbour, then up Victoria Peak for a fantastic view of HK by night. Then finally a stroll through the Temple Street Market.
A truly wonderful day. Have fun 🙂