My wife and I will be in Boston in late Sept. We plan to travel to New York City and return to Boston to meet other family members. We plan to stay in NYC about four nights. What is the best way to get to NYC from Boston and then return to Boston? Rent a car? Take a train? Take a bus? Other options? Cost is a concern.

Look into the cost of Amtrak, i would call them as i've heard they have "Buy one fare, second passenger at 1/2 price" but i don't know how true it is, or if it's only for the summer months. I've taken the bus a few times and the train a few times. Train is DEFINATELY a nicer trip, but the bus is almost always significantly cheaper. The bus isn't terrible, but you're glad when you're off of it.

Bus will probably be the least expensive way. You do not want to do the trip in a rental car. Parking is very expensive and a car is not really needed in NYC.

The Train was fun when I took it. You can also fly.

If $$ isn't an issue, I would suggest the Delta or USAir shuttle. The trai, while costing less, takes much longer.

Amtrak is the way to go.
By the time you get out of Boston airport, fly to LaGuardia and get a ride into the city, you're generally using about the same amount of time as when you are relaxing on the train and watching the sights go by. Parts of the Connecticut ride are along the water, so it's quite pretty.
Bring a lunch with you and you won't have to both with Amtrak food -- just get a drink from the cafe car. I think the Acela is running now. It's supposed to get you to NYC in less than 2 hrs. Don't know how they're record has been tho. Be careful to get a reservation in September, however, as a lot of students who will be coming to Boston schools will be taking the train and it can be quite crowded -- SRO all the way would not be a nice ride in any book!