Otsu and the lakefront
Otsu, at the lake's southern tip, is the most accessible base for a Lake Biwa day trip, with a lakefront promenade and boat cruises departing from the harbour. Mount Hiei rises behind the city, home to Enryaku-ji, one of Japan's most historically significant Buddhist temple complexes, reachable by cable car from the lakeside.
Omi-Hachiman's preserved canals
Further around the lake, Omi-Hachiman is a small merchant town built around a network of historic canals once used to move goods to and from the lake, with traditional storehouses still lining the waterways. Small boat tours through the canal district give a quieter, more intimate version of the canal-town experience than anything in central Osaka.
Getting there
The JR Biwako Line runs directly from Osaka Station to Otsu in around 30-45 minutes, with onward connections around the lake. It's an easy, low-effort day trip that doesn't require the planning a longer trip to Ise or Koyasan does.
Hikone, further along the shore
Continuing around the lake's eastern side reaches Hikone, home to one of Japan's twelve surviving original castle keeps and a landscaped garden alongside it — a natural extension for a day already built around the lake rather than a separate trip in its own right.
How much lake to see in one day
Lake Biwa is genuinely large, and trying to cover both Otsu and Omi-Hachiman, let alone Hikone as well, on a single day trip tends to turn into a lot of train time and not much actual time in any one place. Picking one anchor point — Otsu for the temple and boat cruises, Omi-Hachiman for the canal town, or Hikone for the castle — and building the day around it works better than trying to circle the whole shoreline.