Using Amtrak you'll only get as far as Brunswick and if it's the sticks you're looking for you'll still have to drive for a bit with that rented truck. As for times to avoid I guess it depends on what you're trying to avoid. Avoiding cold weather? Don't visit between November and April. Avoiding tourists at the parks? The reverse is true. But as for activities there's a lot to do during every season. Hope you end up coming to visit and having a great time!