Yeah, yeah, yeah, you could do the typical Mother's Day brunch today, which is nice too, but why not keep it simple and get out in the fresh air?
Grab mom, take her for a coffee and scone at a local Back Bay coffee shop (maybe L'Aroma Cafe) and walk over to the Public Garden. You'll want to get there around 11am for the Duckling Day parade, in which children dressed up as littles ducks retrace the steps of the famous Mallard family ducklings from Robert McCloskey's Make Way for Ducklings, while being led by the Harvard University marching band.
Continue by enjoying the beautiful flower beds and trees through the Garden, then wander on over to take a ride through the lagoon on the Swan Boats, a Boston institution since 1877!
After that, you're on your own! Happy Mother's Day!
(photo credit David Paul Ohmer)