I vote for Denver!! I'm moving there in a couple months
And as far as airfare is concerned... Denver is one of the cheapest places to fly because it is a "hub" for many airlines.
If not Denver... then Monterey, CA
I am going to be selfish in my votes, haha. I am from Monterey county and will actually be spending my summers there. My mom lives there too and has 2 yorkies. She would definitely tag along as well. I think Monterey is a good choice if we go with CA because it is on the central coast and fair distance for SOCAL and NOCAL people. It is also amazingly beautiful. However, It is about 1.25 hours from San Jose, the nearest big airport. So that's kind of a bummer. San Fransisco would also be a good place in CA.