Ganagobie’s otherworldly monastery is an essential stop. Founded in the 10th century, it’s still home to a working Benedictine community, whose products, including soap, honey, jam and beer, are all for sale in the monastery shop. The chapel (worth a visit for its fabulous 12th-century floor mosaics) is the only area open to the public, but you’re free to wander around most of the grounds.
It’s located at the end of a winding 4km lane; look out for signs on the D4096 as you travel between Forcalquier and Sisteron. Note that the monastery is closed during times of monastic retreat.