Lori2 had a question a few weeks back about the watering system above from Smarthome. I will admit to having NO experience with this device. What I do like about it is that it can hold 7 liters and is very clean. It also has attachments for up to 14 plants with the ability to let the soil dry a bit before the next scheduled watering. The one big drawback may be in figuring out how to set the amount of water to be dispensed.
I think that there are also other, simpler solutions for the occasional trip that would not require the use of electricity. They utilize hydrophilic wicks, porous material or capillary mats with a remote water supply.
My three picks for simple indoor watering come from Lee Valley:
These watering globes offer a simple solution that does not rely on electricity.
To tap into a larger water supply I would try these automatic watering devices.
A capillary mat under a pot can also work well.
Another option would be to choose plants that go long periods without water, or need a good soak only once per week (like a phalaenopsis orchid) and like drying out in between. Or something like the ZZ plant that stores water in tubers and can take a lot of neglect.
The CHEAPEST method is using a thick string or yarn to wick water from a bucket to your plant. You dip one end of the string in the water and then coil the string around the base of the plant.
But the BEST method, of course, is to leave a bottle of wine out on the counter next to written watering directions and have a friend stop by while you are away.