Hi Yijean
400 euro for 5 persons, 2 days incl accommodation, food etc? That is not much... Well, of the big cities in Holland, Rotterdam is one of the cheaper ones but still count on at least 25 euro per night per person.
Of course accommodation in the center sounds more attractive but the public transport in Rotterdam is really good so if it is cheaper to be outside of the center I would definitely consider it if I where you.
Indeed as buzzcock says the cube houses are interesting to go to, actually Rotterdam is a major city for modern architecture anyway. The biggest part of it got bombed during WW II and so the most buildings are new and a lot of architects took the opportunity to experiment with all this empty space. The ´swan bridge´ (officialy erasmus bridge) is definitely worth it as well.
A fun thing to do is taking the fast water taxi from one point in the center to the shore where Hotel New York is located (which is a beautiful place to go for a drink, the ships of the Holland-America line used to dock here). A trip with this taxi seems to be about 4 euro per person, and it is real fun because you go with a quick speedboat. Unfortunately the website is not available in English but on the link below you can see the places where the water taxi stops or can come to pick you up. Just call 010 - 4030303 to ask them to come and pick you up.
http://maps.google.nl/maps/ms?msa=0&msid=112830581649792566267.000001126620d2f0e3377&ll=51.916321,4.408264&spn=0.024671,0.068836&z=14&om=1
If you are looking for some green to relax you can go to ´Kralingsebos´. It is has grass, trees and even a (fake) beach. If you start to get hungry you could go to the pancake restaurant over there to get a real Dutch diner (costs between 8-12 euro per pancake - a big one - ).
Last tip ´to do´ is climb the Euromast, the highest tower in Holland. not the cheapest thing to do but the view is so worth it! just have a look at the website: http://www.euromast.nl/english/index.html
bicycles will be for rent for sure, on this website you will find that one bike per day per person is 6 euro. And it is a Kronan bike which is very Dutch and really cool! (and fits a cradle of beer on the front ;-) )
If you dont take the bicycle there are buses, trams and metros in the city so don´t worry about getting around.
Hope this helps, if you have more questions don´t hesitate to ask!
OHannemarie