Seven or eight days feels really rushed for the three major tourist cities in Poland; I'd prefer to choose two. But they are all great, and all have lots of museums to enjoy as well as historic architecture to admire (though Warsaw's rebuilt historical buildings constitute a smaller part of the city). I've been to a lot of Polish museums--not just in those three cities--and am very impressed with how accessible they are to English speakers.
Food in Poland is more varied than you might expect; it's easy to eat well. I confess I didn't eat too often in purely Polish restaurants once I'd tried the savory dumplings and filled sweet donuts a few times. I have a theory that a lot of Poles worked in western Europe when that became an option, a lot of them in restaurants, and returned home to open their own places I've had excellent crepes, Thai food and Greek food in Poland, to give just three examples.
I can't address the availability of sporty activities since I get all my exercise by pounding urban sidewalks. I'm sure there are very good hiking opportunities in the southwest (over toward the Slovakian border), but that is probably a significant travel-time commitment on a short trip.
A agree--another good option would be the Baltic triumvirate of Tallinn, Riga and Vilnius--or two of those three. Although somewhat similar historically (particularly in the 20th century, much of which was really miserable), those three cities are very different architecturally. Tallinn is known for its large medieval center; Riga has one of Europe's largest collections of Art Nouveau buildings (as well as some older architecture); Vilnius is Baroque. All three cities have good historical museums, particularly those laying out the grim but interesting 20th-century period. There were enough art and decorative-art museums to satisfy me, but the art museums don't generally match those in places like Vienna. Perhaps because of an influx of visitors from western Europe, the food scene in the Baltics is quite good. I ate well there without doing much pre-trip research. I basically just at the restaurant ratings that popped up on Google Maps.
I focused on urban sights while I was in Estonia, Latvia and Lithuania; you'll need input from others to learn about rural scenery and activities. I believe it's accurate to say that part of Europe--which is mostly rather flat--doesn't have a lot of super-impressive scenery.
Check travel times between the Baltic capitals before committing to all three on a short trip.
There are also interesting possibilities down in the Balkans, but you can run into transportation challenges down there that make a short trip tougher than in many other parts of Europe. Montenegro is sort of the opposite of the Baltic countries--scenery to die for but not a lot of impressive museums. Because the country is so small, you can get around it fairly well despite the heavy dependence on buses. The national parks look beautiful but unfortunately don't seem accessible without a car.
Albania's getting a good bit of press these days as a relatively inexpensive destination for outdoor activities, and it is justified. It has some really gorgeous scenery and hiking opportunities. But the inter-city transportation is a very serious challenge. I cannot recommend Albania when you only have 7 or 8 days. Maybe if you had a car, but I haven't explored driving times. One reason Albania is so beautiful is its mountains, and they tend to impede your progress when you want to change cities. Keep Albania in mind, though, for a future trip when you have more time.
Romania is great, with cool architecture and nice scenery, but again, fast transportation just doesn't exist, so you really need more than a week there. Because of the size of the country, I'd say the same thing even if you had a ar.
