Rheinterassen

I only discovered this jewel, because of a friend, who lives there. We only went for the "chillout-breakfast", which starts at 10 a.m and ends at 5 pm. This is only open on Sunday, though. They have a huge buffet put up, and for these six hours you can eat as often as you wish. And all for 15 Euros per person.
Okay, this may not give you the kick it did me, but the fun part is, that you can actually leave the restaurant, walk around the pretty Rheinpark and the flowermarket, take a ride in the "tram" (see photo), go for a trip in the "Schwebebahn", listen to something thats going on in the Theater (when we walked by, there was a childrens choir singing) and then go back and eat some more.
The interior of the restaurant is also very lounge-like, since its usually only open in the evenings. They also have an open-air terrace with a pretty view of the Rhine.
There is also a "beach club" close by, with a lot of sand, a beachbar and holiday feeling.
I didn't see it for myself, but apparently there is a great open-air club there too, which opens at 10 pm, I think. Check out their website for more information on those.