At reception they will be given ' A loaf of bread and salt'
That's from the parents, if they're there.
Do poles have stag/hen nights?
Some do and some don't. It's increasingly popular but nowhere near obligatory.
Do they have anything traditional to eat on the day as for eg wigilia?
no, maybe just the wedding cake
i would like to get something a bit more unusual.
Do they have any interests/passions? If not, money is the safest way out.
One thing I can think of is the penny shower - like rice shower but you use pennies. And they're supposed to collect them all.