From Loch Lomond? By heck, that is like driving London to Birmingham, while passing so many places on the way that are brilliant...So ignore this.
You are in the Trossachs, so explore some of that.
Lomond side is nice - a few places to stop and visit, but make sure you do east road as well as west side. Lovely honest cake stall at Rowardennan with lovely autumnal walks for all ages, visitors centre, hotel etc.
The islands on Loch Lomond are worth a visit - not sure of boat timings, but best one goes from Balmaha to Inchailloch. There are also great walks from Balmaha along the shore.
Glasgow is worth a visit - you can do it by train from Balloch. Great city, easy to get around on bus, underground and tram. Lots of free museums and gallerys such as Botanic Gardens, Kelvin Grove, Modern Art Gallery, House for an Art Lover, Transport Museum etc. Good shopping too.
Aberfoyle is a drive over the hill, but worth it. Up the hill is the David Marshall lodge with amazing view, waterfall walk, play areas, cafe, Go Ape etc. Aberfoyle itself is near Loch Ard and the forest park is littered with walks and amazing forest views - feels Canadian in many ways.
Falls of Falloch at the north end of Lomond are stunning after or during rain - worth a wander, and as you go through Glen Falloch the Highlands really open up.
A longer drive (but still not Glenfinnan FFS!) is to Glencoe, so much to see and really really great views. But choose a better day as you want to see the mountain tops, maybe even take the chairlift up at the ski centre.
Stirling is also worth a day out - great castle, wallace monument, Kings Park, free museums and some shopping. From Stirling there is a hill called Dumyat - it over looks the lowlands one way and Highlands another way. A few km and couple of hundred metres up as the car park is high up the hill, but a real adventure and feels bigger than it is. Easy path no worries about getting lost.