Plenty of tracks in Rothiemurchus which may not be a good thing – stuff like the trails round Piccadilly could mess you up!
Loch an Eilein if you can get out there would be great, singletrack round Loch Gamhna is tip top. Or head out to Lairig Ghru and back in. Other option is out past Glenmore Loge towards Ryvoan though TBH – depending on whether you can get transport out there or are under own steam – might not be worth the out and back nature.
Down to Glen Feshie another option but maybe more navigation than is desirable on a time budget. Round Badaguish is another option, little trails there very nice but hard to find, otherwise it’s fire roads in the main.
scotroutes to t’ forum. Or as in Bothy Bikes, they have some local maps for sale..