I stayed at the Park Inn Berlin Alexanderplatz summer last year. I'm not sure how many stars it is officially but I'd give it a solid four. I don't know if they have gym/spa/pool/restuarants since I don't use those usually. The room itself was ultra modern/new/clean. I only paid 90 Euro/night and thought it was a total bargain. It's right next to the Alexanderplatz train stop and right next to the telecom tower (so it's very easy to find your way home if you are lost). Here are some pictures of my room. I highly recommend the place.

http://hw711.com/europe2006/pic/071306/IMG_2626.jpg
http://hw711.com/europe2006/pic/071306/IMG_2629.jpg
http://hw711.com/europe2006/pic/071306/IMG_2630.jpg
http://hw711.com/europe2006/pic/071306/IMG_2631.jpg