Barb, we went interstate when Jacob was 5 months old. We only went from a Friday to Sunday. We packed the nappy bag with twice as much as we normally would - nappies, clothes and all the normal bits and pieces then we had everything we needed for travelling there and back. The bag was still ok to have as carry on luggage if you're flying. I'm assuming that you are going for a week so I would buy nappies when you're there rather than packing a weeks worth.
We took Jacob's car seat with us and used that to feed him in rather than taking something extra. I was still feeding Jacob but took some formula in an air tight container for the just in case - those formula tin lids aren't secure and boy it can make a mess!!
Can't think of anything else now, but will pop in if I do.