I agree, as usual, with Emily. Rushing through Zion AND Bryce in a day is too much, and only spending one day at the GC is just sacriledge. IF I only had one day, I would do Bryce. It's smaller and more compact. I would do the Navaho loop, or just hike through Wall Street if you're out of time. I love that park.

The Grand Canyon needs a week to explore. Weeks. Months...

I have to do a convention in Vegas every January. Even then, it's never cold in the convention halls. Comfortable shoes are a necessity - walking the floor can be a mile hike. Plus, getting taxis might be tough, so sometimes it's easier to walk around. (But, I'm there for the biggest convention they have, so it might not be so crazy for Interbike)