We are a Polish / English household DS is 3 and DD 17 months - we speak English in the home.
We want both children to be bilingual - we use a lot of Polish phrases at home 'Dobranoc' 'Koham Cie' and we are called Mama and Tata by the children. If the eldest waves to say bye he automatically says 'papa' bless him.
I have started doing some more structured learning with my son using Flash Cards (Numbers 1-20) but just struggling to make it interesting...
We spend a few weeks a year in Poland (heading for two weeks shortly) where we will be talking Polish majesty of the time - And am planning on visiting a few book shops for some more books which I'm quite excited about 
But would love to hear from other parents and fun games / exercises / websites they found useful.