Do you have bikes? You could go to Provincetown on the ferry and ride to the beach (or just uber).

There are a lot of things that it's just too cold for...

Axe Throwing, Paint Bars, Candle Pin Bowling, Go Karts, Indoor Amusement/Recreation Centers

Aquarium and Science Museum are standbys. The Arnold Arboretum is nice for a long walk. A walk along the Charles River.

If you like colleges, you could walk around Harvard, MIT, BU, Northeastern, BC.

A walk though Boston neighborhoods might be nice. North End, Charlestown, Back Bay, etc.

Something more challenging would be a hike in NH if you are equipped and experienced to do so at this time of year. I do not know of the state of the ski slopes right now - we've had a lot of snow in the past two weeks but it has also been unusually warm.

“Or anywhere in MA.”

Lol dude that’s a broad category. Where are you even from? If the day is somewhat mild you can just walk around the city and pop into places. If it’s freezing cold I suggest doing something indoors, unless you don’t mind the cold. Cross country skiing is pretty fun. There are plenty of entertainment centers around, like Apex, Level 99, bar arcades, etc. Just use Google my friend.

Don't stress about the downvotes. This sub has a lot of people who think /boston should be Boston people talking to other Boston people.

You picked a bad time of year for outdoor activities. We're in the dead of winter, which means anything outdoors is probably shut down for now. You can go hiking in any of a number of spots, but those might be kinda rough when it's so cold out.

That said, there's still a lot of fun stuff to do in Mass. There's the Minute Man National Historical Park (a series of small museums covering the opening battles of the Revolutionary War) and Boston's Freedom Trail if you like to walk. For more indoor activities, there's the New England Aquarium, the Boston Tea Party Museum, the USS Constitution, Bunker Hill Monument (it's not outdoors but climbing to the top is a bit of a workout!), Hammond Castle, the Eustis Estate, the Witch House at Salem (call them up for times, as they're open less often during the winter), the Salem Witch Museum, the Isabella Stewart Gardener Museum, the Museum of Fine Arts, the Museum of Science (lots of museums in Massachusetts!), and the JFK Presidential Library, among other options.

“you picked a bad time of year for outdoor activities” I didn’t choose when my girlfriend’s birthday would be 🤣 But i appreciate your recommendations very much — thanks!!

That's fair, and you're welcome! Next time you see her parents, though, you should ask them if they can shift her birthday to the Summer. Would make for much more outdoorsy birthday trips to New England! ;)

Go to Logan airport and go some where warm.