This is the time of year to visit Fresh Pond in Cambridge. It’s by far the easiest place around to find Canvasback and there are often other interesting ducks as well. On the whole, I don’t like the place but make a trip or two each year.
I normally park along Huron Ave as it’s the closest point for me, but there is (or was, not sure about current access) some public parking at Neville Manor and you can always park at the mall and work your way over. Cambridge residents can park near the waterworks (and can also let their dogs run free unfortunately). Those entrances are much closer to where the birds tend to hang out if you want to drop in and not walk all around. Other than circling the pond, Lusitania Meadow often has good passerines.