Oh dang, I could really stand to learn where the key bathroom pitstops are around Boston. I usually will drop into a cafe and pray they have their bathroom open, but were some times last year at the height of the insanity when I'd hold it for 4 hours plus.
And yeah, I parked at the end of Vassar where it meets up with Memorial and did a little walking in both directions but kept it short because I decided to not pay for parking lol.
Vassar is good because it becomes free after 6. Memorial is good because both sides are free, and the boathouse areas are free as of 5:31 (half hour parking until 6, then infinite).
Bathroom tips:
Hyatt for sure, parking on Vassar.
MIT at this entrance on Mass Ave
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Faneuil Hall
South Station
North Station
Pru Mall
Longwood Mall (by Food Court)
Seaport Hotel (across from WTC)
Behind the Asian food court in this building in Porter
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Assembly Square right here
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Bunker Hill Musuem Basement in Charlestown
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Piers Park in Eastie has 2-3 private bathroom stalls.
That covers a pretty good area. Otherwise I usually lean towards hotels over cafes. Another good one since you have your car is the McDonald's off McGrath Highway at Twin City Plaza. Also if you park on Cambridge Parkway behind the Royal Sonesta (best place for free parking if you're going to a Garden event) the Science Museum and Cambridgeside Galleria food court are both close by.
Obviously, I have been at this for a long time. Nothing ruins a good adventure more than having to take a leak! At this point during Covid, I often go into the woods (locally to Boston I hit Fells, Blue Hills, Squantum in Quincy) but will be fully vaccine covered by June and can return to public society! I hope this helps! I also come in from the 'burbs!