I travel to China alot for work, but havent been to Beijing. Most hotels have a massage service and they normally offer extras, hotels are more expensive then massage shops but provide a bit of peace of mind. I am sure Beijing will have massage shops everywhere, they can be like 7-11’s at every corner! You will just have to suss out the shops first.
Just a word of warning, massage scams are prelevant in Shanghai, i am guessing Beijing would be the same and i would avoid anyone approaching you on the street offering cheap massages, getting these offers after dark is not uncommon once you walk outside your hotel. I would never accept one of these offers, you can google shanghai massage scams and find lots of bad experiences.