Don't travel too far - from Loch Lomond you are spoiled with fabulous places.
How about a real adventure with some ferries? I find a ferry really makes a trip special...
From Loch Lomond head over A83 down the Mull of Kintyre. So much to explore in that area which would fit with your nice places and historic - from Inverary, to Crinan, Tayvallich and Tarbert. There are some castles (Inveraray), lovely autumn colours starting, the Faerie Islands, good seafood, huge woodland walks and waterfalls, some really out the way places but within 30 miles of Glasgow as the crow flies.
From near Tarbert you can get the Claonaig-Lochranza ferry onto Arran. Arran is stunning, and you can easily lose a few days there - from Brodick castle & gardens, through to lovely villages, great beaches and really, really stunning rocky mountains. Even if you are not hillwalkers, a walk up Glen Rosa is just amazing to see the views and hear the stags rutting calls.
Lots of accommodation options on that loop as well, it is relatively toursity.
Then main ferry from Brodick to Ardrossan back to mainland, and home through Ayrshire roads.
OR,
Stay inland and head east - so from Loch Lomond head to Pitllochry via Loch Tay. Pitlochry is a great base for an autumn week - there are the biggest trees in the UK, high mountains, the remote and wild (and my favourite place in autumn) Loch Rannoch, the Falls of Bruar, Coronation bridge and walk from Pitlochry, the longest Glen in Scotland (Glen Lyon) with the best cafe in Scotland, 'easy' hill such as Ben Vrackie, Dunkeld is rammed with history, and you get to drop into Stirling on the way home which has a fabulous castle and history.