1,5 hours by car You will find the most popular climbing area of North Germany - the Ith (Weser-Leine-Bergland) with excellent sport climbing and good boulder facilities which is sited south of Hannover between Hameln and Hildesheim
about 1 hour south of Bremen You will find near Ibbenbüren some crags - e. g. Dörenther Klippen.
visit the german Website
www.kletternimnorden.de
Although it is only in German You will find lots of beta about bouldering - incl. photos. Ralf the webmaster will probably give You more detailed beta.
Top boulder crags: Pferdestall and Biwakdach (topo via the mentioned Website aviable)
In the Harz there are also heeps of boulders - You will have to drive a longer distance though
Bouldering near bremen