If you are on an escort tour to Great Wall and Ming Tombs, it is very likely that you will visit the silk shop, which sells lots of products made from silk. The shop will also introduce you to what is silk, how to make silk threads, clothes and blankets etc. Some of the silk stuff here is worth buying because of the good quality and it will be more expensive elsewhere, especially outside China. One of the item worth considering (very sellable item) is the blanket made from silk which is supposed to be very good in keeping your body warm during sleep (ranging from RMB$380 to RMB$550 each). Also the silk cloth and clothings are worth checking out.
If you are going for an escort tour to the Great Wall and Ming Tombs, the local guide will most likely bring you to a jade shop located nearby. This is also the location of your lunch so it is almost unavoidable. They will give you a briefing of what is jade, different types of jade, how to carve jade and of course they will bring you to a big shop selling all sort of stuff made of jade. You can look around and see whether you like anything, but it is not cheap as compared to another places (but at least you are assured this is the real stuff). The good thing is that the sales girls in this shop do not try to pester you to buy, unlike some other places in China which I have encountered.
There are several small souvenir shops inside the Ming Tombs, but the souvenirs are quite limited as compared to the Great Wall at Badaling. Since a day trip usually involves visiting these 2 attractions, it would be better to buy your souvenirs at the Badaling Great Wall, but remember to bargain hard (30% of the price or less).
As you might expect of a major tourist attraction, there are many souvenir shops at the entrance of the Great Wall at Badaling, selling various souvenirs and local crafts. The important thing here is to bargain hard, you should start with 30% of the price or less. Try not to buy expensive stuff (e.g. jade) as you cannot be sure whether they are real (most often they are not).