Dinner in Beijing?
Where should I go this coming weekend?
One of the roast duck places. Do a search. I think Quanjude (the original one) is closed, but that is about the only thing I think is a must-eat in Beijing---roast duck.
It would be helpful to know if this is your first time, what you're looking for, and how much you're willing to spend, whether you're coming from outside of China, and whether you're familiar with the range of Chinese cuisines.
As a resident of Beijing, I can make loads of recommendations, but without knowing the above detail, it's hard to make one that's tailored to you.