It's still cold here in Maine, at least north of Portland. We had 3" of snow in the Bangor-Bar Harbor area yesterday (4/3). Today's high is 36F with a "real feel" of 26 due to the wind chill. For the next 10 days the temps will range from 40F to mid-50s, but with 10-15mph winds, so it'll feel colder, and even colder on the ocean. I LOVE Bar Harbor (used to work there)/Acadia/MDI but this is not the time of year to think about whale watching and stuff. In fact, late April up here? I am not sure what will be open in BH, certainly not any of the ocean-going "watching" tours, and not too many shops, restaurants, etc. It's still late winter!
If I were doing this tour, I'd only go as far north as Portland. It's a great little city, lots of good restaurants, and you can go out to Portland Head Light and wander around. Lots of microbreweries in the area, too. Check online to see if any of the nature-watching boats are running yet. Definitely hit Ogunquit and walk the Marginal Way. Shops and restaurants are mostly open now (far warmer and more year-round than Bar Harbor) and you can see Nubble Light in York. Just south of York is Portsmouth, NH, a quaint little town for poking around in. Everything is definitely open in Portsmouth; it's not a seasonal place at all! In Mass there are all kinds of things IN Boston, but in the outlying cities you could visit Salem and take in the Witch culture (and the Peabody-Essex Museum), or go to Old Sturbridge Village is Sturbridge (outside of Worcester). There is also Plimouth Plantation on Cape Cod. There are places to go on the Cape, but once again, it might still be "off-season" so no guarantees of what is open.