I'd spend at least an hour up there. Take photos, grab a drink -- chill out and rest. You have to pay to go up so might as well get your money's worth. Check out all the rooftop pool's while you're there.
The only place to get Chicago style pizza. Some say Geno's East (not a big fan), some say Giordano's (not a bad option), but Pizzeria Due is tops. Except a long wait. Call or put in your name and stroll around the city.