My Favorites
1. Boston

Boston
Highlights: City, universities, history
Suggest Staying: 1-3 days
Stay Around: Downtown Boston or Cambridge
2. Cape Cod

Cape Cod
Highlights: Beach, bay, boating, seafood
Suggest Staying: 1-3 days
Stay Around: Cape Cod
3. Walden Pond

Henry David Thoreau
Highlights: Walden Pond
Suggest Staying: 1/2 day
Stay Around: Boston
4. Springfield

Dr. Seuss
Highlights: Basketball hall of fame, Dr. Seuss sculpture garden
Suggest Staying: 1 day
Stay Around: Springfield
5. Mohawk Trail & Shelburn Falls

Shelburn Falls
Highlights: Nature, scenic byway, falls, small town
Suggest Staying: 1 day
Stay Around: Springfield
Other Considerations
Rockport & Gloucester
Not too much to write home about, but nice little towns along the coast that add some stops along the way if you're roadtripping up through the greater parts of New England.