This is low-cost and convenient. You can able to start at any location, but my family preferred to jump aboard on the first trolley stop at the Aquarium. The Boston Harbor is a scenic area to wait for the trolley, too. It's a great way to see the city so you can decide on the another day where you want to spend the most time. The stops are well-marked.
The drivers were good with Boston history and were humorous at times, but when it rained hard mid-day he (forgot his name!) didn't stop to put down the clear tarp-like blinds. At the nearest intersection at the traffic light, my family got up and wrangled putting down 2 large covers. Alot of people got drenched, and became very uncomfortable with sitting on damp seats and with damp clothing.