A man visiting Ding Dong Dell is rather flustered after misplacing his bally diary. However could this have happened?
This errand is available to complete after completing the main story, as well as completing Errand 084.
In Ding Dong Dell, talk to the man outside of the Cat's Cradle to... ugh, find his missing diary once again. FOR THE LAST TIME! Leave Ding Dong Dell, then start walking directly southwest for an orange and yellow set of trees quite a ways east of the Golden Grove (which appears to be roughly halfway between the Golden Grove and the beach where the Sea Cow is docked.) If you go through the trees, you'll find a path that wasn't there until now, so follow the path a long while until you wind up at a hidden forest glade in the Southern Summerlands.
There's some treasure here, so open the purple chest to your right first to find a wizard king's secret, then make your way to about the center of the area, just to the east of a large mushroom, and you should see an exclamation point appear over Oliver's head. Examine the ground to find the traveler's diary, then go to the dead-end to the west and check out the vantage point here to open the green chest for a star-eater's fang.
Before going back to Ding Dong Dell... notice the large door here to the north. Talk to the... bunny (?) here, who will introduce himself as "The Conductor." Talk to him and agree to undertake his task to unlock a whole new set of errands (See Errand 131).
For now, once you've at least started that errand and have the diary in hand, return to Ding Dong Dell and claim your prizes and extra formula.