I really enjoy the atmosphere at Migas in Nali Patio (Sanlitun), but their drinks are not cheap. I haven't yet, but I plan on checking out the rooftop terrace for the Yunnan restaurant on Beiluoguxiang soon (Yun Er Xiao Zhen).

there is a 5-star hotel named 皇家驿栈酒店 （sorry ,have no ideas about its english name ), near the Forbidden City, the roof terace bar is great !  you can have a good view of the Forbidden City, especially at dust , my favorite !!!! 

I like that mexican restaurant/bar at Sanlitun. You probably know the one I'm talking about it has a sombrero that you can wear and they usually leave the windows open. You have to walk through a corridor and then find yourself standing inside a square with various restaurants. It's straight back you can't miss it. I'm not sure but it might be called Migas. Can't remember the name at the top of my head it's been awhile since I've been back there. 

Dessert Cafe on Gulou Jiu Da Jie. Lovely cheesecakes and great with a beer in the summer (though it's not really a bar I guess.)