Cotonpal, if you can get them lambs' kidneys are very mild, with very little of that distinctive kidney flavor. I have a couple of recipes where I think, if one didn't know it was kidney one wouldn't guess.
I'm not sure about brains since Mad Cow Disease showed up, but I wouldn't mind trying sweetbreads. Oddly (to my mind) oxtail also seems to find it's way onto the "offal" list. I'Ve never understood why.