How about…
Aviemore- ski road – back road towards Boat of Garten – turn right to loop in forest, passing Loch Garten – rejoin back road just outside Nethy Bridge – East to join Grantown-Tomintoul Military Rd – Tomintoul – NE through Glenlivet – Follow Avon to Ballindalloch – Cross to N of Spey just downstream Ballindalloch – Grantown – Carrbridge – Aviemore.
About 100km, interesting mix of forest, moor, river, farmland. Mostly quiet roads, good visibility, plenty of villages and towns with decent café stops. Heading out beyond Tomintoul offers some decent climbs if you want a longer day out.
Or Carrbridge, up roand Lochindorb and along river Findhorn (loads of options) back via Grantown, Nethy Bridge and Boat of Garten…
Or on the old main road through Kincraig to Kingussie (as stevious says, can be extended with an out and back to Newtonmore, Laggan and beyond easily) cross the Spey onto the back road by Ruthven barracks, then follow the back road past Insh, Feshie, etc etc (and explore loops up the various dead end roads to the South) all the way to Grantown, turn back through Carrbridge etc…