hi, I suggest take the bus and go to platis gialos beach, it is a very nice beach with crystal clear water and sandy. This beach is situated 4.5 km, south-east of
Mykonos. This is an area where you can spend your short period of time may be 3-4 hours and then take the bus back. Most of the buses run every 30 minutes to and from the beach and town. When you get back to town, just walk through all of the small streets and watch the windmills and little churchs. It can be done in 3 hours including dinner.. If you go inside the 2 or 3 stores, you will have seen everything that all the stores have, so no need to look around all the stores.
Hope this would help you plan your activity for 8 hrs in Mykonos, have a nice day!