Once you come out of the Siq, there are many local people who will offer to let you ride a camel, or at least take a photo. I didn't do this, but several in our group did. Obviously, there will be a fee if you opt to take the ride, but you should offer to pay if you take a picture with the camels. I paid the owners $1 USD to take these photos of the camels, and they seemed very appreciative.
Not really my cup of tea but if you are keen you can explore the area around the Treasury on the back of a camel. Negotiate your rate in advance! You can also just sit on the camel back and take a picture with the Treasury in the background which will set you back around US$ 2.