PolskiBus is not so close to center. Just check the map. Trains are very close.
True, but in Warsaw it stops at Metro Młociny stop just near the metro. Going to the center from there is extremely easy, takes 15 minutes at most.
In Wroclaw it stops at the main bus station and in Cracow just near the center.
As to the trip, wow.... that's a tight schedule.
I'd suggest you skip a few cities.
IMHO a must are: Warsaw, Cracow and... one of the two either Wrocław or Poznań. I myself will vote for Wrocław (as a good Wrocławianin myself ;)
Gdańsk is cool to, but as someone mentioned the getting there might prove relatively problematic right now. (Unless we're talking by plane). On the other hand you could try figure sth out.
Toruń falls in this category too. Besides it's the smallest of the cities, though its old town is arguably the most beautiful.
So, I'd recommend Polskibus for a roundtrip Warsaw-Cracow-Wroclaw-Warsaw.
I took Wroclaw-Warsaw (6 hours) and Wroclaw- Cracow (3 hours), so you won't waste time.
In Warsaw the communist architecture combined with the cosmopolitan skyscrapers will be something quite unique for you, I bet. It's a former seat of Polish kings (as was Cracow) so you absolutely must see the King's Castle and the Old Town. See also £azienki park and the Warsaw Urising Museum-one of the most innovative museums in Europe where you can really feel history coming back to life.
In Cracow...too much to name. Wawel Castle and the Old Town are a must. Sukiennice will be a nice place to buy souvenirs. Nearby you'll find also the Mariacki Church, an absolute miracle of medieval architecture and art.
If you can have a slight detour from the biggest cities, I'd suggest going just outside of Cracow to see two things:
First, a solemn reminder of the German-Nazi Holocaust. The Auschwitz concentration camp.
Second, Wieliczka salt mine. This is a one of the wonder in the world--over 700 hundred years old salt mine with underground artwork, sculptures and a whole frikking underground church made of salt. You wouldn't believe your eyes. This will be a memory to cherish for years!
From Cracow and thereabouts go to Wroclaw. Here you can see Ostrów Tumski-a medieval island covered with churches, Old Town, The Wroclaw University museum with astrological tower observatory open for the public. And last but not least be sure to spend an evening here Wroclaw has a great nightlife scene, have something to eat in Piwnica Swidnicka, the oldest operating restaurant in Europe. Oh, and starting in May are multimedia displays on Wroclaw multimedia fountain. They are worth seeing.
Let's say you still have time, go visit Poznan then (I'm not familiar with Poznan's sighs, but I hear it's cool).
Just my 2 cents,