I know there are some great local trails in the area, I remember my first XC race in the Scottish series was at Aberfoyle, great course!
Also, there are trails heading northwest over to Callender, and from there you can follow the Sustrans trail up the side of the Falls of Leny. There's loads of great rootsy singletrack in the trees between the Sustrans trail and the river so keep your eyes open.
Once you reach Loch Lubnaig, there's a short anti-clockwise circuit up into Stank Glen with a brilliant wee descent back down to the lochside.
On the other side of the road and slightly further up is a forest road which climbs towards Glen Ample (check an OS Map). The Glen Ample track is a famous route although it all got washed away in a crazy storm a couple of years ago and the replacement estate track is a bit meh. If you wanted to tag it onto Stank Glen/Falls of Leny though, you could ride as far as the highest point then turn around, retrace your footsteps so far then turn right and descend the steep and techy hillwalker's trail back to the lochside.
Heading southwest from Aberfoyle you can follow forest tracks to Drymen and tag onto the Loch Lomond Section of the West Highland Way. This would make a good ride up and over Conic Hill and along Loch Lomondside as far as Inverarnan, at which point you could winch your way back up the road to Loch Katrine and enjoy a gradual tarmac descent back to Loch Ard.
Lots to keep you busy