We stopped in Boston for an afternoon on our way home from Cape Cod. As it was a quick stop and our first visit to the city, we thought we'd see the highlights with a hop-on-hop-off tour, and found this one on Groupon.
The tour covered some pretty key spots, and the guides on the trolleys we rode on were both informative and entertaining. We've done this type of tour in several other large cities, and the guides we had were impressive.
I can highly recommend this service, especially if you're trying to acclimate to the city before venturing out on your own, or are trying to see a lot in a short time like we were (with young kids, too).