Sustainable Vegetable Garden

The best answer to this question will be the most insightful one. I’d like to improve my suburban, California garden to be productive, organic and sustainable. What I’m really asking in this question is what sort of investments would give me the most bang for my buck, particularly in the long term. The suggestions could be for plants, tools, infrastructure, assistance, and so on.

What I want out of my garden:
Food (fruits, veggies)
Exercise
Satisfaction
Something to look at besides a flat, weedy, square back yard

What I already have to put into my garden:
A 50-foot square back yard, of which about a third has been cleared of weeds and the process of soil improvement begun.
Five existing fruit trees
Various seeds, already saved or otherwise acquired
A library card and an Internet connection
A gardening staff of one small woman (me)
Basic tools, including two compost bins