You'll be fine by yourself. The subway is very easy to get around on (and cheap by our standards), everything you really need to know is in English. In terms of needed a guide for attractions, you don't need one, but at some places it can be useful to have a good guide as you'll learn a lot you wouldn't if you were on our own. We recently did a walking tour of some temples in the city center and we could easily have walked around by ourselves, but the guide told us much history and other things that we never would have learned ourselves. Ditto the Forbidden City. Our guide explained many things and also knew routes that let us avoid some of the major crushes of people. The great wall you can certainly do by yourself.