Yes it violates the terms of a Transit Without Visa at PEK. For a day trip, it's unlikely that you would come up against any officialdom that would ask to inspect your passport and find you're in violation. You will have to show your passport to buy tickets, but clerks only look at the info page to type in your name/number, never check for visa or TWOV. As said above, you must return for the night to Beijing to stay. Don't get into trouble where the police or PSB might want to take a close look at your passport, but do take it with you on your day trip.

I can only theorize as to what the authorities would do with a TWOV violator, since I haven't heard of any instances where this has happened. Presumably they could black mark you in their computers and on your passport, which might affect future attempts to pass through or get a Chinese visa.