I often find pianos in the hotels where I am staying...sometimes in a ballroom, a meeting room, or a lounge. More often than not I just sneak in and start playing. Every now and then a worker will pass by and say, hey that's nice! If the piano is locked you can ask at the desk whether someone will unlock it for you. There was only one time when a gruff desk attendant came and asked me to stop playing the piano in the lounge. I asked him why and he said "if you play it the piano will go out of tune and then we have to pay to have it tuned!" I told him the piano will go out of tune whether you play it not...and in fact it desperately needs tuning right now!
If you travel a lot for extended periods you might consider buying a lightweight keyboard (perhaps with only 66 or 48 keys) to take along with you. I used to have one that fit in the overhead compartment and so I'd take it onboard the plane.
You could go to piano stores and spend a half hour "trying out" their wonderful pianos. You can keep your repertoire fresh that way!
Since you're now in Boston you're surrounded by dozens of colleges, universities, music schools, and conservatories. Most of them have student lounges and public spaces furnished with pianos. Just drop in and have your way with one.
This last bit is not terribly ethical so use it only in truly desperate situations: Many music stores have a 14 day to 30 day return policy on keyboards. So if they won't rent you one, just buy one! Try it out for a couple of weeks and then return it! Uh, uh....I didn't tell you that.