branch office
I went to a seminar on this once, organised by a law firm who specialise in it (don't remember the name, however if you want I can PM you with a name of a lawyer who was there and can point you in the right direction). There's still quite a bit of paperwork to register a branch office, however it's easier than, say, registering a limited company in Poland.
There are some tax advantages, however you should be aware that the tax office do pay special attention to people who've registered companies abroad to trade in PL.