Went to 6 countries in Eastern Europe in September so my recommendation, if you are looking for a city is undoubtedly Budapest. There'd be plenty to do for a whole week even if you didn't leave the city itself. (If you do think about it check out a hotel called Hotel Normafa in the Buda hills - a great not too expensive spot). Croatia is nice (Split, Dubrovnik and the coast rather than the slightly dreary Zagreb), but we actually preferred Slovenia. Went to Ljubljana which is nice - small but worth a look around for a day - and then to Bled, which is where Tito used to have a villa. It's a lake near the Julian Alps, not too far by train from Ljubljana. There's a beautiful castle overlooking the lake, a monastery on an island in its centre, and a national park nearby with a fantastic walk along a gorge. It might be a bit too late in the year for all this depending on the temperatures though, but it'd be worth checking out. Bratislava would probably be cheap enough to get to too - we got Ryanair there from Dublin for very little - but it tends to be full of drinking groups at night. Could be that we hit it on a weekend but it ruined the beauty a small bit. Sarajevo too has plenty to do but it'd be a bit further afield I think if you're only going for a short time without wanting to travel too much.
If you're willing to think a bit about central Europe (which is how I'd define Prague, and I'm sure the people themselves would too!) then Austria is one of the best places I've ever been. Vienna is a fantastic city, if more expensive than Budapest, and Salzburg is also beautiful.
In any case, do make the journey further east, it's worth it (though so is Paris!)
Edit: I just noticed that you said for next year so I'd highly recommend Bled then in that case!