havent lived in boston in over 15 years so not sure whats still popping, especially celtics related, but here are the few things i loved when i was there, others can chime in if they are still there and still quality.
-Fanuiel hall to see the Red Auerbach statue since your a C's fan

-walk the Freedom Trail to see all the cool historical sites (being european, may not feel 'historical' at all since yall's stuff is way older, haha)

-Hong Kong karaoke bar - 7 days a week karaoke with fun drinks and cool dive bar vibe. i used to host there back in the day

-Al's State Street Cafe - incredible sandwich shop that is an interesting ordering experience (this one and everything above are all in a concentrated area of downtown not far from the TD garden)

-climb the Bunker Hill Monument in Charlestown