2 good routes setting off from Aviemore as below
1. – Cycle out ski road then out to Loch an Eilen from Rothiemurchus cafe.
At the end of the Loch turn off to the Cairngorm club footbridge below the Laraig Ghru.
Continue up to Loch Morlich.
At Loch Morlich cross the road and up to Ryvoan bothy then descend down to Loch Garten (visit the Osprey centre if you have time). Follow the road back to Boat of Garten (pint in the pub beer garden if the weather is good)
Back to Aviemore along the Speyside way.
2-
Same route to start but once you reach the bottom/Start of Loch Morlich turn right and climb up the forest track to Glenmore lodge. At the lodge follow the path on up towards the Lairig Ghru for a mile or so then heather romp across to your right and you will find a really fun single track path back down.
There is then a foot path runs back to Aviemore in the forest parallel to the road
Niall