Water thoroughly right before you leave and then as soon as you get back. Set up a gallon jug above your plant, fill with water, poke a tiny hole in the bottom and leave the cap loose so the water drips onto the soil. Buy an automatic irrigation system that will water exactly how much you want on a schedule you set.
Not a bad option if dealing with soil. I use these guys on indoor grows of basic kitchen stuff and strawberries. Overwater prior to trip and then pop in aqua globes and then when I return a week or so later plants are all good (if either underwatered or overwatered.. still alive though). Never did this with marijuana grow since I do hydro. If hydro would just increase my reservoir.
Aqua globes -- or any other method that keeps the soil continually moist -- are bad for MJ. But it may be better than letting the plants dry out too much for too long. Basically would do the same thing for the plant as the milk jug idea I described, neither is a good method for watering MJ. But the question really isn't what is "good" for MJ but rather what will keep it limping along long enough without killing it.