Bordeaux is a great city to visit - it's a lovely, smaller version of Paris (much of Paris was designed after it's 18th century design), but many come to learn more about the wine culture of Bordeaux. If you only have one day, there is plenty to explore in town. I'd invite you to spend at least two, because visiting a producer outside of town is a must, be it the famous Chateau Pape Clement which is accessible by public transport or the intimate, family owned producer of Chateau Beard la Chapelle in the hills near Saint Emilion.
When I was visiting Libourne a few months ago, I realized I needed to know more and knew just the person to ask - Jennifer Poe of 'My Bordeaux Tours', who is an experienced local tour guide and American expat living in Bordeaux. She is the blogger behind 'American Mom in Bordeaux' and has worked with multiple river cruise companies offering tours to their clients. Libourne has a lot to offer any visitor from walking along the port, wandering through the old bastide section of town, a lovely fine arts museum and exploring it's variety of stores, cafes and restaurants. Especially on market days, Tuesday, Friday and Sunday mornings - this town is alive and vibrant - not just around the holidays.
At the bottom of the hill to Saint Macaire, the Garonne river used to touch the town and boat loads of wine would make their way to Bordeaux. Sadly, the rare Saint Macaire grape isn't grown much in the region anymore, but biodynamic producer Chateau Cazebonne will have their first bottle of 2020 Saint Macaire ready next year!