Oh that’s a tough question.
One day is a very limited amount of time to see everything Beijing has to offer. It would be impossible to see everything that you would not want to miss. We tried to do it in 4 days and only scraped the surface. If I were you, I would probably play it safe and choose from one of these more itineraries that are more focused and tailored to your taste:
Feeling adventurous and don’t want to limit yourself to a single itinerary? Start anywhere in the city (probably wherever you are staying) and just start walking. Do what the locals do, try to take the subway to Lama Temple and spend some time around there. Don’t miss the Wudaoying Hu-tong in that area. We visited NanLuoGuXiang and head north from there to arrive at BeiLuoGuXiang. BeiLuoGuXiang is supposedly busier than NanLuoGuXiang but street food there are super expensive and considered not as good.
Next you can take a bus to Houhai, which is famous for its scenery. Don’t miss the Water Cube at night for its lovely view.
Of course some will argue you can’t miss the Forbidden City if you are in Beijing. Go as early in the morning as possible as there will only be more people as the day progresses. Houhai which we talked about is also in this same area and on your way to the Forbidden City you will likely pass Tiananmen Square on the way.
Guangji Temple on Xisi Road and the China Geological Museum that’s just right across can be two other options. The China Geological Museum exhibits a lot of geological features from around China and should be a stop for anyone who’s interested in geology. Following this transit line you will also find the White Pagoda Temple. Around the White Pagoda Temple are lots of small alleys, and they will look familiar to those who’s seen Beijing Hutongs on TV. Next stop should be YuanMingYuan (the Old Summer Palace) and the Summer Palace.
Remember you aren’t limited to exploring in-town. If you decide to skip these buildings in town and choose to spend a whole day at one of the sections of the Great Wall you will not regret it either. We visited the Mutainyu section of the Great Wall and was really glad with that decision.
In the end, our best advice is to hire a local travel guide and a car if you can. We did that for our time in Beijing and that was an absolute time saver and you need that if you can only spare one day to see Beijing.