Give it constraints
The more it knows, the better it plans. Mention your budget, pace (packed vs. relaxed), dietary needs, who's coming, and anything you want to avoid.
Point to a place or a day
"On day 3", "near Alfama", "after lunch" — anchoring a request in space or time gets a sharper result than a general ask.
Iterate
Treat it as a conversation. React to what it produces: "too packed, cut one stop", "more local, less touristy", "keep the dinner but move it earlier".
Ask it to explain a choice — "why this neighbourhood?" — when you want the reasoning, not just the result.