The Lairig Ghru descent (from the trail heading past Rothiemurchus Lodge) is quite short and can be tacked onto almost any route in the area. Loch Eanich is always worth a visit and is another out-and-back.
Carn Ban Mor (down Glen Feshie) has probably the most bike-able ascent. If you want to make that a longer day, start from Rothiemurchus and cycle past Loch an Eilean, Loch Gamnha and over Moor of Feshie to Feshiebridge, then down the river-side track.
Burma Road is a half-day ride so you might want to head round by Nethy Bridge, Forest Lodge and Ryvoan on the return le (though that part of the circuit is normally better anti-clockwise)