There is plenty to ride around that area. Glenbranter itself has the “Twister” – sections of singletrack the forestry put in linking some of the surrounding fire roads. There is the Loch Eck loop which is around 20 miles mostly easy going on fire roads and is worth a ride, link in to the Twister at the end.
Nearer to Dunoon there are plenty options as well. And some purpose built stuff at Lochgilphead – Fire Tower Trails
Short drive to Arrochar you have the Ardgartan peninsula circuit which is a nice ride, mostly fire road but a decent section of singletrack. Around 20 miles.
Glen Loin loop, shorter ride about 12miles, good section of singletrack.
Park at the walkers carpark at Succoth and go up the hill path to the Cobbler. Tough climb but great descent. You can also add in Ben Ime or over other side of Loch Lomond Ben Lomond is a great descent.
Plenty of options on the nearby West Highland Way.
Get yourself a copy OS Landranger 56 and go explore, mountain biking opportunities are endless.