I was there a month ago. I agree a guide isn't need for the great wall. I Highly recommend Mutianyu portion of the great wall. I personally really liked the Summer Palace, I am glad I paid the extra $ and went up to the top of the temple there. I would have liked to spend a little more time and enjoyed the surrounding park, the locals were playing music and some were dancing. I liked the Temple of heaven and that park as well. I wouldn't do the great wall and Summer palace. you could do a day with a guide and fit in the Forbidden City, T. SQ. and Temple of heaven. I have weak lungs and I didn't have any problems with pollution while I was there, but a lot can happen in a month. BTW a great hotel I recommend highly was the Red Wall Garden hotel.
It was small with personality, very clean, sweet staff, great dinner and comfortable beds.