That is, the first thing to do is to decide what types of things interest the both of you. From there, you can generate a list of "must-sees" and from there, an itinerary. My preference is for summer travel, as more places I enjoy are accessible. But if you're into, say, skiing, winter would be a more opportune time to go. With two months at your disposal, you could even incorporate a couple of seasons. As an example, in northern New England, mid-August and mid-October can be quite a bit different, and each time has its own pluses and minuses.