The pump on timer sounds plausible but there are other ways to do this.
You csan cut back SIGNIFICANTLY on the light while maintaining the same light SCHEDULE, whcih will reduce the rate of transpiration and water use. When I leave for a few days I kill my big K HPS and use just a 400 mounted VERY high, and take half the tubes out of my flouro fixtures until I get home.
You can also make a wick system, with a piece of that silky nylon rope with one end in your pot, buried pretty deep, and one end in a big bucket of water.
Experiment before you leave if you have time.