You don't say what mileage you consider to be a "moderate hike", but you would be walking part of the West Highland Way along the east shore of the Loch, either Balmaha to Rowardennan or Rowardennan to Inversnaid or Inversnaid to Ardleish.
From Ardleish there is a little ferry across the Loch to the Ardlui Hotel. Finish your walk with a snack at the Hotel then catch the Citylink bus back to the parked car at Tarbet. The stretch from Inversnaid to Ardleish (or on up the canal to Inverarnan) is surprisingly strenuous.
https://ardlui.com/hotel/#ferry
If the weather was nice I would personally choose the Loch Lomond walk.
You would need to drive up the west shore of the Loch to either Luss or Tarbet then park there (I would choose Tarbet over Luss)
https://www.cruiselochlomond.co.uk/waterbus-ferries
The waterbus to the now unpopulated island of Inchcailloch with time to explore the island is also interesting, and a much gentler alternative.
You could decide on the day depending on weather. If it was raining you could do Stirling Castle then the 2 hour drive to Oban via the A84 Callander and Lochearnhead as far as Killin, then the A85 to Oban.
This is a video of the Balmaha to Rowardennan stretch (starting from Drymen further south)- https://www.youtube.com/watch?v=liPkz47s-CI&list=PLt9UiadU1aXl_cnDaIpDM5r1dTmh220SJ&index=2
and this of the Rowardennan to Ardlish for Ardlui (and on to Crianlarich) stretch-
https://www.youtube.com/watch?v=Ynhtyzah_-4&list=PLt9UiadU1aXl_cnDaIpDM5r1dTmh220SJ&index=3
This is the Loch Lomond Faerie Trail (and new nature trail) mentioned by Mike, above-
https://www.lochlomondfaerietrail.com/
It is interesting to see Lorna's videos (Jo's companion) of the same walk on her channel- Lorna Jane adventures.
A totally different possible visit on the drive to Oban is the Cruachan Hydro Electric Power station- https://www.visitcruachan.co.uk/