Do you have live plants?
If you do not have live plants, then definately keep the lights off while you are away, or else you will return to the algae from hell.
If you have live plants, a timer is a must. Live plants will wither and die without lights, and only make a messy tank for your return.
Feed your fish very well, at least three times a day, before you leave. Fish can survive without food for a good week without ill effect, provided they are well fed and healthy before you go.
Do a small volume water change just before you go, as Gnome said. Make sure you leave yourself enough time to monitor for problems that may result from the water change, do try to do one as you walk out the door because some emergency may happen that you won't be present to correct.
Just make sure your tank is completely stable before you leave and everything should be fine. Unless of course you have a power outage, which happened to me once when I left for a three day vacation and came back to a tank that had been without a filter all weekend!
~~Colesea