Portugal + what other countries

Is that 11 days total, door-to-door including flights? And is that flying in from far away (North America or equivalent, on an overnight flight)?

If yes and yes, then you get 8 or at best 9 actual, usable days in Europe. If that's the case, then I'd just stick to Portugal. Because you would burn a day flying from Lisbon to your other destination and one more day getting back - so then you're down to just 6 or 7 full days when you're not traveling or recovering from travel.

That's 11 total travel days EXCLUDING airfare and what not. We are flying in from the US.

That's 11 total travel days EXCLUDING airfare and what not. We are flying in from the US.

Don't know what "airfare and whatnot" has to do with anything. What I was trying to get at is how much time do you really have, because many (most?) first-time posters here are not being honest with themselves about how much time they really have, and they plan an unrealistic (sometimes wildly unrealistic) number of stops in the short time they have.

You say you have "11 total travel days". Does that mean you have 11 full days, on the ground in Europe, after you have arrived and gotten a good night's sleep, not including the day you arrive and not including the day you depart? Or is "your trip" 11 days long, from when you leave your home in the US to when you return back home? The two are quite different.

I'll ask the question another way: what are the dates on which you leave home, when you arrive in Europe, and when you fly back? Without knowing this, it's hard to tell if you have 9, 10 or 11 days to play with. And on a very short trip (which is what yours is), that makes a difference in how many places to go.

There is plenty to occupy you for 11 days in Portugal.

I’ll assume for the sake of argument you have 13 days vacation and are not counting the day you arrive (a lost day of jet lag and fatigue) and the day you leave. So, if I’m right and you really have 11 full days of travel you still should stay in Portugal. Maybe not what you want to hear but there is so much to see and do. You will find that transportation to any of the other places you listed, even Sevilla which looks tantalizingly close, will take a day from your trip. Look for a train from Portugal to Sevilla and you’ll see what I mean. And, even though it’s a short flight, you really want to hassle the many hours it takes at an airport? Look at the RS Portugal tour, get the book, plan from there. We took that tour and wished we had a few more weeks in that wonderful country.

I agree with others that there's plenty in Portugal alone to fill 9 or 11 days. That said, if the OP really wants to add another location, that could be done, but only -- and I stress only -- if one would fly into Portugal, and then departs from that second locations -- for example, Madrid. By going from Portugal to [anywhere] you burn a day lost to travel. Maybe that's OK. But if tickets are booked flying in to Portugal (presumably Lisbon) and the return flight is also booked departing from there, forget it -- that would burn two full days of the trip. Only if one booked open jaws (say, in to Lisbon, and out of Madrid) might it be anything approaching reasonable IMHO.

Root of the problem is, this is just too short a trip, period.

OP should also note that December = short days, so that's another factor.

There's nothing around Portugal but the sea (and Spain, but conections are really bad). So I think you might take a plane and visit Miami! Not so far away! Sure you will like it!

We spent nine days just in Lisbon and then traveled throughout Portugal for a couple of more weeks. If you must leave, Sevilla makes sense but travel between them is not easy. The islands would be easier to connect to.

Check out the weather for Madeira for December - there’s a 50% chance of rain each day. Others have already given you good advice about the difficulties of adding another location and the time lost getting to and fro.

There is plenty to keep you occupied for weeks between Lisbon and Porto. Are you travelling over Christmas? Travel and accommodation will get booked up early.

We spent 14 days in Madeira and Porto Santo in the beginning of September and enjoyed it a lot - but in December I am not so sure.

There is a lot to see on Madeira, but Porto Santo is mainly for relaxing.

Thanks everyone! Yes, 11 days total, feet on the ground. Excluding travel time, not airfare :)
Yes, we will be in Lisbon during Christmas.
It sounds like staying put in Portugal and exploring its other fine cities is a great option.