Thanks everyone for all your comments/suggestions. All very very useful.

We have decided to fly to Boston and spend a few days there, then potentially hire a car (depending on price) and spend a few days driving to New York, then stay in New York for a few days then fly home from JFK. If the hire car is too expensive then it'll be the train.

I have 3 weeks off work so taking it slowly and spending plenty of time in places won't be an issue.

It'll also be our wedding anniversary so I think spending it in New York would be ideal.

Now we can look forward to researching places to stay, places to see and places to eat.

Thanks again!