pieces of advice for young travellers (Duesseldorf)

Hello, my son and his girlfriend chose Dusseldorf and its area to spend a few days on holidays in february Protected content 24). They are 18 years old, and not too much money... (thank you Ryan Air !)
could you give them some pieces of advice, and tell good places you tried... :
where to sleep, what to see, what to do ? My son is speaking a little bit German, and his girlfriend speaks rather good english.
thank you for your answers !

Catherine, from Provence