State and nature parks alongside the northeast USA

Hi there!
I am currently planning an itinerary for a 15-day roadtrip in the northerneast part of USA, beginning and ending in Boston. Our vague plan is to head east to Maine as far as to Acadia National Park, then possibly head a further north, and then to ride alongside the north border through New Hampshire and Vermont to Niagara Falls, before we head back.

Can I ask for some tips for any wonderful state parks, national forests, mountains, or other nice places, mostly in the means of nature, scenery, rivers and so on on the way? Usually, we like to pick a spot for about 2 days, walk some easy to moderate trails in the area and then move to the next stop.