so I am going to Disney very soon and I needed a bag that I could clip on to the stroller to carry our stuff in. I found this great tutorial on this website: http://www.makeit-loveit.com/2010/09/2-in-1-bag-stroller-bag-into-messenger.html
Here is the finished product: (well almost... I still have some buttons and velcro to add.)
This first picture is with the long strap out for use when your bag is off the stroller.
Then you tuck the strap in and use these small straps to clip on to the stroller.
Here is an example of what it looks like size wise for easy carry. Fairly simple!