Marseille

I did the Frommer’s walking tour of Le Panier and enjoyed exploring the area, though some of the walking directions were unclear. I walked around the vieux port. I took the bus up to the Basilique Notre Dame de la Garde for the beautiful views of the town and the sea.

Hi,
You can easily fill up a day in Marseille wandering around the port going all the way to Fort Saint Jean (do go up / inside) and Mucem (no need to visit the museum itself if the temporary exhibition is not of interest to you), backtrack into the old town (Panier district) to wander around the old streets (it's a small area), pop into the Vieille Charité and have a light lunch somewhere, work your way back to the inland end of the port and up to Noailles and its market, with an interesting Middle Eastern / North African flair.
In August you might need to be somewhere air-conditioned in the hottest hours of the day, but it's all so close, so you can easily go back to your hotel.
Later in the day, ideally close to sunset, you should definitely go up to Notre Dame de la Garde for the views. And get a drink al fresco on the Cours Éstienne d'Orves (large square near the port). If you have some spare time, you should also head to the "Corniche" by the sea - specifically the Vallon des Auffes area, quaint and with great sea views.
The following morning, I'd take a taxi or Uber - expensive (55-65€) but at 6-6.30AM (your hotel will advise what time) it will be much more comfortable than dragging yourselves to the train station (which could require a cab anyway depending on how far to the metro you are staying) and taking the shuttle.

Thank you for the suggestions. It was helpful and definitely looks like easily a full day.

There's too much to do it all in a day, but I also give high priority to a boat tour of the Calenques, departing from the center of the Vieux Port.

https://www.visite-des-calanques.com/en/

https://www.bleuevasion.com/calanques