I take my D-reg aircraft back to Germany for it's annual each year. I have been into Bremen itself which was very friendly but make sure you are familiar with the VFR arrivals and departure procedures including VRPs. The field I go to is Bohmte-Bad-Essen which is grass and to the south of Bremen.
I regularly land at Osnabruck-Atterheide to clear customs and this field is really friendly and well equiped. I have also used Damme.
Points to note are that the controllers english is often limited away from the really major airfields and nobody seems to understand QFE but will give QNH.
Transponder with mode C is mandatory (at least for D-reg aircraft) and the standby squawk is 0021.
Have fun.