Shore dive to the Wreck of the "Liberty". Because of the stone beach a little careful water entry. Sea bottom further on black sand because of the vulcanic origin. Wreck itself is an oase of marine life. Shallow and broken enough for easy acces and exploration. Plenty of parking is available on site. Air fills are found at dive shops around dive site, but not on the actual site itself. Very crowded during diving seasons (or so the guide told me), but usually pretty good number of divers are around you.
More info
http://www.shorediving.com/Earth/Pacific/Bali/USAT/index.htm