I would be curious to know this as well.
I have large patches in my backyard that needs some greening up but would be too expensive to install permanent irrigation.

Indian Hawthorn (Rhaphiolepis indica) comes to my mind. Although I really prefer the pink to the white flowered varieties.

Try a native California shrub. No or very little water needed when mature, and many of them bloom with beautiful blossoms.

http://www.theodorepayne.org/

I am happy with natives Rhus ovata "Sugar Bush" which makes a full shrub about 6 feet high and 10 feet wide, evergreen and looks good year around with no water and no pest problems. Rhus integrifolia "Lemonade Berry" grows as multitrunked large shrub to about 15 feet, upright about 6 to 8 feet wide. Also evergreen needing no water and having no pest problems. I have had both for at least 15 years here in the hot end of the Napa Valley. Al

I suggest mountain lilac (Ceanothus.) It comes in various sizes/shapes. Dark Star has lovely flowers, though it may not get as tall as you like (I forget the height.) You can read about some Ceanothus options here:

http://www.humeseeds.com/efcean.htm

If you are in southern California, I suggest Matilija's or Las Pilitas. I think they both have websites.

Another option is coffeeberry.

Why not browse the Las Pilitas website and see if they have something that strikes your fancy? Many natives also attract butterflies, another plus.

Here is a link that might be useful: Las Pilitas

Las Pilitas and Matilija's are native nurseries, not plants.

How about Buddleia? There are so many to choose from and they are readily available at most nurseries. I grow Nanho Blue and Honeycomb but plan on growing a few more. They make an excellent screen, are drought tolerant and are a bushy shrub. Added bonus - they attract butterflies, which your grandkids would probably enjoy seeing. : )

Google buddleia and you'll get lots of info.

Diana

Pomagranate, Wonderful or Granada variety. This is one tough plant that is GORGEOUS in the spring with its red and yellow blooms. My favorite ornamental shrub, actually. Left unpruned, pomegranates grow into dense bushes, 6-10 feet high. After a year or two of watering, poms will be able to survive with very little water. The fruits are highly bird resistant and delicious. I've seen poms planted in freeway medians and thriving in Lodi Ca.

Forgot to mention that pomegranates drop their leaves in the fall. Only downside.

I'd offer up two, the taller Crape Myrtles and Photina, specifically Photina Fraseri. The latter has new growth of red leaves and whte flower clusters in early spring. This is a fast growing bush and doesn't mind lack of water once established and I would call it drought resistant being through several with it. Crape Myrtles are drought resistant plants and ones can be found that will grow anywhere up to 5-10-20 ft tall. There are dwarf varities. Crape Myrtles can be found in colors of red, purple, pink and white and shades thereof. Also grows pretty fast.

Fall is a better time to plant. It's difficult for a plant to get established at this time of the year, no matter how drought tolerant it is.

Coyote brush (Baccharis pilularis) is a nice native shrub for slopes. Low, dense, stays really green.

Here is a link that might be useful: Baccharis info