The small island is only 20 km square and is visible from quite far with its Thai temple built at the top of a hill. Driving up there is easy, and a small road drives around the temple, surrounded by many small shrines dedicated people passed away. Park where you can and walk to the temple to enjoy a beautiful bird view on Phuket town.
Koh Sirey temple also shelters a golden reclining Buddha which without being impressive, is still as big as the temple. A note: the old guy standing by inside the temple will request a donation if you want to take a photo of the reclining Buddha, this is most unusual, and he is pretty grumpy, so better drop a small note in the donation box (It’s a nice gesture anyway).