I am surprised to read the advice above. It is very typical to do the Temple of Heaven and Great Wall in the same day, especially for people who are only in Beijing for a short time.
I still support the idea of doing the ToH in the morning. You can head for Mutianyu between 10h and 11h at which time the rush hour is long over (although there is always a fair amount of traffic in Beijing).
You will get to Mutianyu sometimes around mid-day which gives you more time than you need to see it. I recommend doing the Great Wall in mid-day in the winter in any case as any ice on the steps will have melted by then and it won't be as cold as going early in the day.
I guess that it all comes down to individual opinions but I have been to the Summer Palace a few times and find it boring and I assume that it would be even more so in the winter.
People used to combine the Great Wall with the Summer Palace on the same day but that was before the subway to the Summer Palace. It made sense to have the driver take you to this attraction which was out of the way and without proper public transportation. No longer the case but geographically it is sort of in the same direction as Badaling if you choose to go there. Badaling and the Summer Palace make sense as a combination.
Personally, if friends were visiting, I would never ever suggest a Summer Palace + Badaling day. But hey, we all have our opinions.