I've stood in for Ian_Giles quite a few times over the years. He gets me to do his shops for him when he goes on holiday (and he has lots), but I never really like doing them since I don't know what shop is where and all the little 'funnies' mess me around.
Personally, I think you should just let your customers know that you won't be around during your holiday dates, and they'll be fine with that.
We're only the window cleaner; they'll cope with not having their windows cleaned for a short period, I'm sure.