In reply to DR:
Glen Einich is good for a half day. A while since I did it but I remember it being a fairly gentle uphill gadient all the way to the loch and a nice fast descent all the way back. One river crossing which shouldn't be a problem as very little rain forecast this week. Combine it with a circuit of Loch an Eilean on the way back. Badaguish has some interesting singletrack. So does Inshriach/Glen Feshie but that may be a bit far for a half day. Some info here: www.mtbtrails.info
Cheers Tom