Loch Lomondside has some great riding, up to Rowardennan the West Highland way is relatively straightforward singletrack, after Rowardennan up to Inversnaid it gets prgressively more interesting, with some nice rock and root features to negotiate. In fact, the ride from Rowardennan to Inversnaid and back would make be a nice little route.
On the other side of the Loch, its a short drive to Arrochar, where there are two good loops, either round the Ardgartan Peninsula (starts very easy fireroad but gets better, and the diversion to the waterfalls above Lochgoilhead is really good).
A shorter loop would be to head north from Arrochar through Succoth and ride the forest road up to Sloy Damn, there's a really nice singletrack descent back down the other side of Glen Loin.