How do you get from North Station?

I'll give you two ways, depending on your tolerance for walking. The Aquarium is pretty close to the Green and Orange lines. I think you'd spend the same amount of time underground as above ground, so I just walk it.

Take the Orange Line Inbound to State. Go upstairs, and walk down State Street about 1/4 mile to the Aquarium. OR, at State, change to the Blue Line Outbound, go one stop to Aquarium.

Once you are done at the Aquarium, wander over to Faneuil Hall/Quincy Market. It's a great food court on steroids, and the most visited place in Boston. Go out to the front (street side) of Faneuil Hall. If you are OK with stairs, stand with your back to Faneuil Hall, look for the stairs ahead of you, and go up to Government Center. If you don't like stairs, stand with your back to Faneuil Hall, turn right, and walk a block to Haymarket. Either way is fine.

Haymarket and Government Center are both Green Line stations. Take any inbound train except E to Kenmore, get off and be swept along with the crowd to Fenway Park.

From Fenway Park, reverse the trip. Go back to Kenmore, take any train that says North Station.