Polterabend - help

[no_touching]

So, I have my first Polterabend in a week. I've read the wikipedia article, so sort of know what to expect from that. Still, I don't want to make any social faux pas, so any advice for a complete n00b? Is it quite formal or a big piss-up? What kinds of gift are typical? I really don't have any spare plates - can I really take a terracotta flower pot? Just in case it makes a difference:
- NRW
- young (mid-20s) couple, living together for about a year, still studying
- we don't know them that well, but they have housesat for us several years in a row