Not sure where you are starting, but here is my $0.02.
Duesseldorf is a must in my book, and you can get your fill of Altbier by doing short walks between the brewpubs. Last time we just went to Zum Uerige several times. You are close to Koeln (Cologne), so you can check out Koelsch, and the Dom (Cathederal). Take the slow train that follows the Rhein River to see a metric ton of castles through the Rhein Gorge.
Muenchen is super in the summer, as the biergartens are in full swing. If you decide to go there, get "The Beer Drinkers Guide to Munich", yes that is a real book. Munich has many museums and historic sites if you are into that, and the Nymphenberg palace, and the Residenz (SP- my German is gone), along with some good churches.
The last two time in Germany we have spent several days in Bamberg. That is beer heaven in Germany.