In Fiesole itself there’s a very pretty forest walk out to Montececere, which is where Leonardo and his crew performed the first experimental test flight back in 1508. There’s a small marker at the spot and a picnic table nearby if you want to pack a lunch and enjoy the scene. Along the way there are nice views down to Florence below, and the route passes by an ancient marble quarry which, according to local lore, is where Michelangelo supposedly selected some pieces for his sculptures. It’s only about a mile and a half out, and most of it is an easy stroll thru a leafy wood.
You can also walk back down the hill into Florence - there are several small side roads that avoid most of the vehicle traffic.