IMO, you're painting Manhattan with a ridiculously broad stroke.
You could easily spend a day in Central Park or the Museum of Natural History or SoHo or Battery Park or even Midtown (Times Square). It's Disneyland for adults.
For an eight hour day, I'd recommend hitting Griffith Park and hiking to the Observatory (which is unbelievably physically demanding). Once you've reached the top, taken pictures that, on a clear day, reach 100 miles or more of ocean and mountains, head down to Santa Monica, rent a bike and ride on the Pacific Ocean bike path to Hermosa Beach.
Once you've reached Hermosa Beach, go to one of several beach side restaurants and/or bars, drink to your heart's content. Call a cab back to your hotel, put the bike in the trunk and return it the following day.