If you have 9 days in eastern europe in late august (coming from NY), where would you recommend going, poland, bulgaria, romania, slovenia, or montenegro?

been to czech republic, slovakia and hungary already

You could easily spend 9 days in Poland, or you could spend 9 days in either Romania or Bulgaria (or a combination of both). It will be difficult to include either Montenegro or Slovenia on an itinerary that doesn't include either the Czech Republic or Hungary, but if you wanted to visit Croatia, you could easily combine that trip with Slovenia.

But August is the worst time of the entire year to go. The weather is likely to be hot, everyone in Europe will be on vacation, so prices will be at their highest and room occupany (and airfares) at their highest. Most hotels in Eastern and Central Europe do not have a/c, so be aware of that.

If you postpone your trip for a couple of weeks, it will be infinitely more comfortable and less crowded.

I would do Poland, Slovakia, Romania, Bulgaria. You should be able to spend most of the trip in the mountains, which will be much cooler, especially in the evenings. The mountains are quite stunning.

I might stretch Poland into East Germany which will be relatively empty in late august.

Montenegro will be packed, go North young man

Poland, you can easily spend 9 days there, stay along the Baltic coast if it is hot then head south to the Tatras but Doug is right, expect crowds in August. Schools are on holiday throughout Europe.

A Fodor's Top 10 Must-Do adventure you can find here http://www.visit-transylvania.us/holidaycalendar.html (trips in ROmania and Hungary)