Cycling
Is probably the best outdoor activity besides hiking. the lowland highland trail, which is part of the national cycle network route 7 starts from balloch, going north to drymen, aberfoyle, callander, strathyre, lochearnhead and killin. free leaflets with a map of the bike trail can be picked up at tics along the trail. there is another cycle path along the western shores of loch lomond. bikes can be hired at the loch lomond shores see above.
Hiking
Is also a good idea. you could for example go to rowardennan and then hike up ben lomond. the long-distance west highland way also runs along the eastern shore on its way from milingavie near glasgow to fort william.