Faneuil Hall has a bathroom in the basement, and I believe there's some in Quincy Market, too. The Marriott on Long Wharf has a public restroom that you can access from the outside, on the side facing Columbus Park. They were doing maintenance on it recently, though.

Redi in Newton, by the Eliot T Stop, is nice.

Go with 2.5mg or 5mg, maximum. Don't be tempted to take more, because it can take hours to kick in and varies by person and product.

You can get ones that have a blend of THC and CBD, which is really mellow and uplifting. Otherwise, any indica options should be chill as well ("indica = in da couch"). Enjoy!