Time with your nieces in the capital, plus a loop of the meticulously rebuilt Old Town — a UNESCO site precisely because of how completely it rose again from wartime rubble.
City greenery worth the detour: Łazienki Park, the royal park of pavilions, peacocks and red squirrels around the Palace on the Isle, with free open-air Chopin concerts on summer Sundays. The wilder eastern bank of the Vistula is a protected Natura 2000 corridor running through the middle of the city.

The Old Town, founded in the 13th century, was deliberately razed by German forces after the 1944 Warsaw Uprising — around 85% of the historic centre was destroyed. Between 1948 and 1953 it was rebuilt brick by brick, partly from Bernardo Bellotto's 18th-century vedute and pre-war surveys. UNESCO lists it as an 'outstanding example of a near-total reconstruction of a span of history.'
Google-rated 4★ and up. Tap a name to open it in Google Maps.
A hotel in or beside the Old Town. One night.
City charging is plentiful (public AC and DC hubs across the centre). Museums to pre-book if you want them: POLIN and the Uprising Museum both sell timed tickets.