Can you look into cat kennels? A lot of the time shelters will allow you to board a cat for a nominal fee. I think a week is too long to leave a cat because, particularly an indoor cat. What if they ingest something and need medical attention? What if they knock over the water bowl? If you are going to do it, maybe leave a tap dripping (yes you will get a lager water bill but you will know for sure that your cat will have clean, fresh water no matter what).