Well-established Slavic country with colorful Eastern block charms, and without the tourists.
As an American in Europe, I’d say Poland is probably the first truly Slavic eastern-European country you’ll reach. It’s close enough to easily arrive from western Europe (neighbors Germany/Czech), yet far enough to maintain its own vibe and doesn’t have hordes of western tourists and foreigners like those 2.
Poland is truly a gem for me. Kind people, great food, cheaper than the rest of western Europe (but not cheap). And I love that it has a different set of tourists, mostly all European and many from further east and south of Europe…it’s kind of the international hub for them. What you get here is IMO a much more authentic eastern European vibe than Czech Republic (because of its Disneyland tourist presence with bigger old town and charmier bridges/water-points).
5-10 days would easily cover 2 to 3 cities. And give you a full range of Poland’s charms.