when I was looking into this last year, the facebook-consensus pointed me to DKB. There, you get a girokonto with some negligible amount of interest, and a seperate account for a visa card where you can preload money on to it like any other bank account (via Überweisung), and earn I think something like .8% or .9% interest. It is what I use as my Tagesgeldkonto. I'm interested to see if anyone has better suggestions though!
