Most of the "Front Range" of Colorado comes close to meeting his criteria. It isn't a "cool climate" but the heat index isn't bad and winters are moderate. Some places are pricey, but not all. I also like the central Oregon suggestions.

But normally you have to trade off some of these vs the others. For instance living here in the suburbs of Madison, Wisconsin offers several of these benefits. The taxes aren't low and there are no mountains, but there are many wonderful nature areas in the vicinity. There are many, many lakes. And we score very well on the rest of the list.