As Donna mentioned, spending some quality time looking at maps and getting an idea of the many many options you've got in front of you is probably the best place to start. You're very city focused at this point, but the US is filled with so many amazing natural features, beyond the Grand Canyon and Niagara Falls, that it would be a shame to miss out on them.

For Zoos, one of the best zoos in the country is a place you probably wouldn't think to look: Omaha, Nebraska. The Henry Doorly Zoo is always ranked among the best in the US, and even tops several of those lists.