Tough decision!
Hanalei Bay was the location of my favorite open water swim race ever, so if I want to just swim forever, Hanalei Bay.
But I also love Anaeho'omalu Beach in Waikoloa. I could swim there and then go out to the Queen K highway and ride my bike for a few hours as well. Of course, stopping at the malasada food truck that hangs out up near Hapuna Beach.