La Purisima Mission State Historic Park is the strongest reason to stop in Lompoc. The mission was founded in 1787 and rebuilt at this site after earthquakes damaged the earlier settlement; today it is preserved as a large state historic park rather than a single church courtyard.
The scale matters. Restored rooms, workshops, livestock areas, trails, and open hills make it easier to imagine a working mission complex than at many of California's more urban mission sites. Allow at least a couple of hours if you want to walk beyond the core buildings.