The Renaissance Ring is a series of lovely walks outside Florence.
It covers a distance of 170 km and can be done in sections as large or as small as you like.
For smaller ones in the city:
1 Cross the Ponte Vecchio and turn right, down Borgo San Jacopo and simply wander the whole area of Santa Spirito which is the artisan quarter.
2 Cross the Ponte Vecchio and head diagonally left up the tiny & steep Costa San Giorgio. Follow via San Leonardo, left onto Viale Galilleo, then up to the right to the Monastery of San Miniato al Monte. The on to Piazzale Michelangelo, overlooking the city, then down the hill to the river and home along either side of the river. About 2 hours in all.
3 My favourite, Fiesole to Settignano – walk along the hills from one village to the other. http://florencewalks.blogspot.it/2008/06/fiesole-montececeri-settignano.html
Take the #7 bus to Fiesole from Piazzza San Marco. Walk to Settignano, have lunch in the wonderful Caffe Desiderio http://www.caffedesiderio.com/ and take the #10 bus back to Florence.
This is only a tiny sample of what is available and if you want longer, guided or self-guided walks in Tuscany or in Italy anywhere, have a look at what Simon Tancred's Hidden Italy Walks offer.