Perhaps some bikini for the rest of this summer, whatever that is left of it. However summer in Poland is not particularly hot. It's still relatively cold with rain some days. Bring your jacket to ready yourself for the winter, maybe some formal dresses/suit for the formal occasions. Some regular casual wardrobe. Generally for clothes, if there are some things you forget to bring, you can just buy. Maybe some of your favourite books in English, because there are not that many in Poland. Camera to capture some moments if you like.
I wouldn't take bikini, if you are not a very dedicated fan, chances of good sunbathing weather are getting smaller every day. What to take depends what you plan on doing here, and what clothes you have.
Between September and February you may see almost all the weather Poland has ( other than hot summer).
For September you are going to need some light rainproof shoes, light jacket and something warm to wear under jacket (hoodie, sweater, jacket...), umbrella.
For later you need warmer clothes. I know some people send those by mail, to save on luggage limit.
If you have warm jacket or coat, you'll need it (or have to buy here). Warm ankle high shoes. some kind of headwear (hat,cap), scarf and gloves.
