With car:
1) head north of Warsaw on E77 to Gdansk for ca 45 min, turn to 85 and 575. Your destination: south bank of Vistula opposite Zakroczym. First stop Sady, just right of motorway bridge,
2) second stop - Stare Grochale - drive further west, pass the forest and turn to dirt road immediately after forest ends to the right. Drive or walk north short distance to Vistula (both places: waders, waterbirds, migration)
3) then meadows in Kampinos National Park, drive from Grochale on 579 and turn to minor road in Kiscinne, Wiersze. Find one of several dirt roads going north, park before it, walk on foot to meadows along Kanal Lasica (canal), walk along this canal north of villages Wiersze, Brzozowka (raptors, farmland birds).
4) return on the minor road via Brzozowka, Janowek, Adamowek to 77 towards Warsaw
5) then north Warsaw sewage outlet to Vistula (gulls), go to street Prozy and by foot south around the fence, to find rough road going to the river. If you are short of time - skip it. If you have more time - walk north on paths to get to Las Mlocinski forest park. Keep around the flood dyke where broadleaved trees are.
5 alternative) also northern Warsaw, forest park called "Las Bielanski" or "Rezerwat Lasek Bielanski" north of Podlesna street (broadleaved woodland for Middle Spotted Woodpecker etc.),
You need a full day for it, perhaps skip sewage outlet, if you have enough time, try forest in the morning.
This is realistic for many new birds in one day, this time of the year. Skip Modlin meadows (usually empty in late summer). Skip Stawinoga ponds 35 km east (too far/awkward road).