The beach. Unless things have changed since I was in Hawaii last, they're open 24/7/365.
HOWEVER, from recent news reports, you MAY be sharing your towel with some of the myriad of homeless people...the economy there has taken quite a downturn...
http://www.modbee.com/news/national/story/979408.html
I'll be in oahu in Christmas, where to visit there that day when some of the sightseeing places will be closing?