Going on holiday--or vacation--requires plenty of preparation, and if you don't have a neighbour, friend or family member who can water your plants while you're away, you must make concessions for that. Fortunately, a number of solutions all work well to keep your plants watered and healthy while you are away.
Decide if you plants actually need water while you are away. Cacti and succulent plants (e.g. milkweed, purple rose tree) that retain water in their leaves can do without water for long periods of time. In fact, the plants you water only once a week may be able to do without water if you are going on a short holiday.
Water your plants well before leaving. Then, use one of the following techniques listed below to keep your plants well watered while on vacation.
Set your plants in a dish of water. This only works if your pots have drainage holes on the bottom. The longer you are going to be away or the more the plant usually drinks, the larger the dish of water should be.
Mix in some polymer crystals and water your plants again (or put them in before watering as described in step 2). The crystals will swell with water and as the soil dries, the water is released. This works well for outside plants and indoor plants.
Make your own watering wick. Select a plastic bottle size that is appropriate for your plant. It can be as small as 340gr or as large as 1814gr. Fill the bottle with water and the set your finger over top of the opening. Quickly flip it over and shove it a couple inches into the soil. This works well for both inside and outside plants.
Purchase a commercial watering system such as a watering wick or capillary mat. Watering wicks are similar to the homemade one described above, but are professionally made and look like an upside down vase. Capillary mats come in many varieties and are typically filled with water and small holes release the water onto a fabric top. The potted plants sit on the wet fabric top and soak in the water from the bottom.