Sofia - Borovets - Peshtera - Plovdiv

It starts from high up Vitosha mountain, then a long a very pleasent discent into the Samokov valey and thereafter it runs mostly across mountain valeys, along the foothills of Rila nad Rodopi mountains,  Starts with a short climb, then there is a long descent followed by two more not-so-long climbs.  I'd say, it's an easy (though a bit lengthy) route for a one-day trip.