If you're after scenery, not necessarily loco hauled there's a great diesel Iberian gauge line from Puertollano (on the high speed line south of Madrid) to Merida. Merida is a good place to stay for a night, lots of Roman ruins. From Merida you can continue to Badajoz then take the CP DMU service to Entrencamento. This line is also very scenic (and slow), but will eventually be superseded by the new High Speed line being built from Lisbon to Elvas so worth a ride.
I'd recommend Bobadilla to Ronda and beyond. If you avoid high season the DMUs are quite comfortable and people tend to spread out despite the RENFE compulsory seat reservations which bunch everyone together!!