old rag is nice, but always too crowded as has been mentioned.
I like jeremy's run trail in SNP, I go in at the bottom of the mtn near rileyville, va and go up. there's a nice smaller waterfall there and other than humping up over the first hill, its a pretty easy hike. There are enough trails in SNP to keep you busy for quite a while though. I believe th eSNP website has information on them.