A school bus that played a key role in the 1960s hippie movement has returned to San Francisco. Used by author Ken Kesey and his Merry Band of Pranksters, the “Furthur” bus is now part of an exhibition celebrating the Grateful Dead. Opening Saturday, “Forever Grateful, Golden Gate Park” will also feature Jerry Garcia’s BMW, never displayed in public. The exhibit immerses visitors in the counterculture of the 1960s and invites them to explore its meaning today. At a time of political polarization and collective fatigue, the exhibition aims to connect people through art, music and activism. It runs through Oct. 25.