I go from "Wildwood Flower" into "Under the Double Eagle" and then sing "Columbus Stockade Blues" then maybe "Freight Train" (the Libba Cotton one) if my fingers aren't too tired by then (rarely).... all in key of C (though usually capoed 2nd fret).

A few others to look at: A Fool Such As I, I Don't Hurt Anymore, Blue Moon of Kentucky, Folsom Prison Blues, and a Willie P. Bennett instrumental called "Summer Dreams, Winter Sleep" (I usually use that one as a warm-up tune to limber up the fingers).