There’s a campsite on Loch Morlich. There’s a variety of trails on the doorstep, and the north end of the Lairig Ghru is a couple of easily doable miles away. Up to where the Sinclair Bothy used to be from the campsite and back would be a decent enough day with wild forest and out into the mountains. PLUS, there’s a canoe/kayak hire place n the loch and the river that runs into the loch just next to the hire place looks like it needs exploring.
EDIT – I’ve never heard of the Cairnmgorm Loop, but that looks good, bothies at Choie Etchacan and Corrour FWIW