I’d do the route that highlandman advises, i was up that way in glorious sunshine for 10 days in April of this year – the route he mentions is a belter of a run – very fast in sections out past Ryvoan bothy especially on a cross bike.
Any route up that way is doable on a cross bike, depends how many times you wish to try your luck on the rooty descents.
I took the path up the hill behind Glenmore lodge and dropped down to An Lochan Uaine and then continued on the Ryvoan bothy circuit, the roots were easily manageable on my Cove Hummer with 140mm forks the previous day but decidedly tricky on the Tripster, i made it with a few graceful dismounts though.
Also check out the paths from Aviemore -> Loch An Eilein -> Loch Gamhna and then take the upper path past the Bothy and down past the open peat bog area into the Nethy forest and play around on the numerous paths and finally make your way to Inshriach nursery for cakes and coffee (essential), return by any number of tracks back to Aviemore.
Pop into Bothy Bikes in Aviemore and ask for the Trail map set, I’m sure David and the boys will look after your needs.