In reply to tony:
Depending on your definition of "a decent day", I can thoroughly recommend the following circuit :
Start (say) Loch Morlich, along past Glenmore Lodge and An Lochain Uaine, take the right fork and climb up the shoulder of Bynack More, take the lefthand fork at 775m, and follow this path all the way to Fords of Avon, and on down the Lairaig an Laoigh and Glen Derry to Derry Lodge. Then follow the path west and then up over the Lairig Ghru, and down to Piccadily. Turn right here, back along to Loch Morlich.
It's about 32 miles, with approx 1150m of climbing - I've done it about 4 or 5 times, and loved it every time !
You can cut through the Chalamain Gap, rather than going down to Piccadily to shorten it slightly.
Usual caveat about conditions, regards crossing Fords of Avon.