Yerba Buena Center for the Arts is across the street from SFMOMA. The park is a lovely oasis of calm in the heart of the city. In the park there is an awesome fountain/sculpture honoring Dr. Martin Luther King: the viewer walks underneath a pounding waterfall and as they walk they read King's words of his vision for the time when justice rains down on us like a mighty waterfall. It is one of the best-done memorials to a visionary leader I have ever seen. Walking along, hearing the pounding water fountain, even getting a faint spray of water, and reading his powerful words is a powerful experience.
The Yerba Buena Art Museum is, in my humble opinion, uneven. It seems to have no permanent collection and in experienced curators. Or maybe it's mission is to focus on obscure, unknown, hopefully-emergent artists who are not yet doing truly museum quality work? It's always interesting to go in but with so much great stuff to do in SF, altho I am often right there, I usually skip the museum.
It's a great spot in the city, tho. If you don't know it, check it out, esp. the Martin Luther King memorial.