Hi, all, and thanks for the helpful responses!
Yes, we'll be going later this month, mid-week. I'm hoping for passable weather for walking.
I hadn't heard of Codzilla... and after checking out the website, I'm especially disappointed it doesn't operate the day we'll be there.
The Aquarium is a special point of interest because my nephew has never visited one before. So we might take a water taxi (to Rowes Wharf?) straight from the airport, find a quick bite, and tour the Aquarium.
From there, we could walk to the Children's Museum/Tea Party Ships area. That might just occupy us until an early dinnertime and return to the airport via Silver Line from South Station. But if I sense he's not up for another museum-type experience just then, is there a good place to let a kid run around, like a park?
I'm still figuring all this out, and yes, hours-long one-on-one care and feeding of a four-year-old is going to be a relatively new challenge for this uncle. The suggestions here are much appreciated, and I'll check back here for more ideas as the big day approaches. Thank you!