Staufen is a lovely town to visit on a Sunday. You can take the train from Freiburg via Bad Kronzingen or the bus from Freiburg. The train station is opposite a lovely ruined castle that is easily accessible on foot via some vineyards. On they way down you can stop off at the inn and taste the wine produced from the grapes that you've just walked past.
Going further afield, Titisee used to be very pretty but has been spoilt by tourism but the journey there is worth the trip. Feldberg and Belchen are also worth a trip up as the views are amazing on a clear day.
If you have a Konus card you can get a small discount on the Cable car rides and free rail/bus travel.