The phrase alone didn't make sense until I searched where it was from. I found it in a Family guy episode. With the full context as such: 00:08:13 What a God-awful mess those things are, chocolate with raisins in it. 00:08:17 Yeah, yeah, that's what kids want with their chocolate-- fruit. 00:08:20 Why don't you put sunflower seeds in the Ding Dongs while you're at it? 00:08:23 - Is that part of your stand-up act? - I don't know, do you like it? 00:08:25 I wouldn't open with it. From this scene, It seems to mean that chocolate and raisins are as terrible of a combination as ding dongs (a sweet American chocolate cake treat) and sunflower seeds. As an American, I wouldn't find sunflower seeds on ding dongs appetizing. (I've​ attached an article with pictures for the treat) https://en.wikipedia.org/wiki/Ding_Dong

Hey there. "While you're at it" in general means "since you are already doing X activity, you should also start doing Y". The rest of the sentence supports this with it's structure and context very well. Ding Dong is penis (or part of a penis?), so basically this is a sarcastic sentence suggesting a person to shove up sunflower seeds into his "penishole", because he is doing something just as stupid already... I hope i'm correct. :D

It simply means why don't you do something extra, while you are doing something. For example: Why don't you plant the sunflower seeds while you're planting the roses. Why don't you clean the shower while you are having a shower. Why don't you take out the recycling when you take out the rubbish. You could rewrite it as "Why don't you put sunflower seeds in the Ding Dongs when you are doing that?".

You usually say it to get someone to do something, when they are going to do something else. Example: Go to the store, and while you're at it, pick up a movie too please? Drop the kids off at daycare, and while you're at it, could you buy something at the store for me?