What about the gel substance that holds water? (Originally it was in a crystal form, but it comes in gel now, too.)I can't think of the name, but they would have it at a garden center and an employee should know the name of it.

Plus, if it's a house plant, you could water it well in the tub (let it soak for a little while, I believe) and then put a plastic bag over it and tie it shut. (Wouldn't want to use black, though; you want light to reach the plant.) The condensation will collect on the inside of the bag and then water the plant in your absence.

This method makes you move the plants while you are gone but I have used it & it works. Use a childs swimming pool or a large tub...place bricks in it & fill with water just over the bricks. Place the plants on the bricks & water 1 in. around the bottom of the pots. This will give them the mosture through the bottom of the pot. This works for short term, 3 to 4 days. I haven't tried it for any longer. Has anyone tried the sponge method? I mean by that, cutting ordinary sponges into small pieces & mixing with the soil. Suppose to make the sponges hold water & plants draw on it when the soil dries. Betty

Here's a link for one you could make to water during vacations.

Here is a link that might be useful: homemade self waterer

I collect all of the plants in one location, and set up a small sprinkler that I use just for this purpose. An inexpensive automatic timing device can set the sprinkler to come on when I want it to for as long as I want it to.

I have, though, recently seen some brilliant waterers while flipping through one of my seed/plant/gadget catalogs. I remember thinking. "Now why couldn't I have invented this?"
I'll see if I can find it.