Sit in Piazza Navona and see the street art there
Climb up to the Hermitage above Assisi (and take the correct path this time)
Walk to Lago Albano to go for a swim
Climb the Dome of St. Peter's
Walk along the Tiber
The Uffizi Gallery, just a train ride away to Florence to see Botticelli's Birth of Venus and Primavera.