Are your lights on a timer for the trip? If so, I would recommend reducing the amount of time they are on. Lighting seems to be the most common culprit I come across when people are having algae issues, along with overfeeding/excess nutrients. Is someone going to feed while you are gone? If so, I'd recommend either showing them HALF the amount you normally feed (in my experience, they will overfeed anyway), or getting one of those weekly pill containers to put in the amount of food to eat each day.
Also, if you haven't read the Co-op's algae guide, give it a read.
Hope you have a great trip!