Second destination either before or after a week in Paris - Lisbon? Budapest? Somewhere else?

Why not Budapest, as you mentioned ? It's wonderful going there in the winter as the cold outside is incentive to cozy up inside at great restaurants, cafes, and at the thermal baths. There are direct flights between Paris and Budapest.

If you liked Prague, it will give you a little different flavor.

Budapest is definitely a top contender! Any suggestions on navigating the city as a solo traveler with zero grasp of the language? I'll of course try to learn hello, goodbye, please, and thank you, but I have limited expectations of my language ability beyond those key phrases.

These suggestions are pretty obvious I suppose, but if you haven't visited them before (although I'm guessing someone named Colleen flying from Boston may have Irish roots and have done so!), two cities with direct flights from Boston, plenty to do for 4 days (especially indoor things like museums and theater that are good for December), good for solo travelers, quite different from Paris, and with good onward connections to the City of Light, would be London or Dublin. I don't know about inexpensive hostels, and London especially is quite expensive for accommodation, but they both have a wide variety of different types and price levels of accommodation.

Also agree Budapest is a good choice.

I have visited Budapest solo 6 times and while I have learned a bit of the language (I am 1/2 Hungarian) I have never had an issue communicating in Budapest--if you venture to other towns it can get interesting but I must say the Hungarian people are lovely & very kind and helpful, and they are very much delighted when even a feeble attempt is made to speak their very difficult language.

Surprisingly, Slate, I have not yet been to either Dublin or London. I did really enjoy a trip 2 years ago to Glasgow and Edinburgh, so Dublin might be a good option.

Christa, thanks for the feedback on Budapest. It seems like such a fascinating city, and the baths would be amazing in the winter.

The Christmas markets would also be open in Budapest at that time. It would be a lovely contrast and 4 nights (3 days) would fly by. But I have never tried a hostel so I can’t speak to cost or quality of sleep. I do stay in apartments, which are cheaper than hotels with lots of choice.

Edit: I am on my 4th solo trip and language is no issue. I can manage hello and thank you in Hungarian.

Thanks for your input, TexasTravelMom! I'm interested in hostels both for the cost savings and for meeting new people. Any tours you can recommend for that second piece?

I appreciate all the reassurance about not knowing the language!