Just leave everything you don't want outside your front door and within minutes some scavenger will take it and put it to good use elsewhere.

If you want to donate your food/clothes/furniture in Beijing just go to the chinese red cross. They will take supplies and distribute them according to people's needs. That's what they are there for besides other things. I worked for the American Red Cross and the Chinese Red Cross follows the same guidelines as others around the world do, If you can contact the people at the Chinese Red Cross they should be able to help you out with your belongings that you have no need for.