Rotterdam has various well and lesser known museums worth a visit!

Modern art:
Boijmans van Beuningen Museum, www.boijmans.nl
Museumpark 18-20, Rotterdam

Kunsthal Museum, www.kunsthal.nl 
Museumpark, Rotterdam

Witte de With Centre, www.wdw.nl
Witte de Withstraat 50, Rotterdam

Other:
Havenmuseum (Harbourmuseum), www.buitenmuseum.nl
Leuvehaven 50, Rotterdam

Nederlands Architectuur Instituut (Dutch Architecture Institute) www.nai.nl
Museumpark 25, Rotterdam

Natuurhistorisch Museum (Natural History Museum) Rotterdam, www.nmr.nl
Westzeedijk 345, Rotterdam