Perhaps one of the most notable projects is The Sea Ranch, a 5,000-acre planned community on the coast of Sonoma County, California. Created in the 1960s by Halprin, architects Charles Willard Moore, Joseph Esherick, and others, The Sea Ranch has come to define a rugged brand of California modernism.
Other important works include the Franklin Delano Roosevelt Memorial in Washington DC, which opened in 1997, 23 years after Halprin won the initial design competition; Skyline Park in Denver, Colorado; and numerous public spaces in San Francisco, California, such as United Nations Plaza, Ghiradelli Square, Levi's Plaza Park and the splendid Stern Grove Ampitheater.
Perhaps my best experience with Halprin's work was in 2007 at The Sea Ranch, when Dwell's editors, designers and photo team took a creative retreat there, a place that deeply speaks to what we do as modernists based in California. Though the meetings all went well, none of us could wait to get outside for a walk on the bluffs or amidst the tumble-down fences, old barns and weathered geometric forms that dot the landscape. It seemed that Halprin's genius lay in leaving so much untouched, or perhaps it's that he concealed his fingerprints so thoroughly.
Just a few obits have come out so far. John King, the San Francisco Chronicle's able design writer, has a brief summation of Halprin's career here—though keep an eye on King; he's bound to do more.
Architecture for Humanity's Cameron Sinclair has also remembered Halprin here, and calls him "Modernism's Olmsted."
For further reading I suggest you go to Halprin's oeuvre, as he was a fairly prolific chronicler of his career. I'd start with 1995's Sea Ranch...Diary of an Idea from Spacemaker Press not only for its lucid view of the place, but for Halprin's wonderfully expressive watercolors as well (left). You might also visit the Lawrence Halprin Collection, which is held by the University of Pennsylvania but can be perused online here.