I know it is probably a bit touristy and a bit cliche to visit Tiananmen square, but it is also the heart of the heart of China.
I was a child of the 80's and while I don't remember much about 1989, I remember images from Tiananmen. It is moving and awe inspiring to stand where so much history has been made. It is something I recomend to everyone plus it is close to just about every major attraction in downtown Beijing.