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.
Best zoo in the world. Take the zoo bus, there are two types, ride the one you can jump off and on throughout the zoo. The other just stops at a very few spots.
Definitely take some time to walk around the mall. It's quite large, but I'd recommend heading over in the direction of the capitol, or going over to the Lincoln Memorial and Vietnam Memorial area.
Unless you are really running low on souvenirs emblazoned with male genitalia (and who isn't?) and/or non-descript leather bags, there's no reason to allow a lot of time here. It's a tourist trap and a dull one at that. Rub the nose of Il Porcellino for luck and kicks and be done with it.