You can see the longest coaster and the highest waterfall in Germany just going by the bus from Freiburg. It takes about an hour to get there, but the ride is free, when you have the Semesterticket. I suggest to do these 2 things at once, as the attractions are in the same area so you will only need like 5 hours (with the ride) to see both things.
How to get there (I did so):
Take the bus 7215 from Freiburg Bus Station at 11:35 (this bus goes once a day but it's a direct connection) or bus 915 to Titisee and then take bus 7300 to Todtnau. From the bus station in Todtnau it's about 5 mins walk only to the coaster- you will definately see it. The ride costs 10 EUR (the ride up the hill included). Once you are up the hill you can go 10-15 mins to the observation tower before you will ride the coaster. It's really worth it!
The waterfall is indeed in the same town but coaster is in the valley and waterfall at the hill. That's way you need to take the bus 7215 (direction Kirchzarten) and then take off at stop "Todtnauber Hangloch, Todtnau" REMEMBER that bus stops on demand so you just simply need to push the "stop" sign in the bus and it wil stop. The entrance to waterfall is only 2 mins from the bus stop (you need to go in this direction you just came from- just a few step back then). Then you will see the entrance. There's a clear path so don't worry you won' get lost. After you ' ve seen the waterfall stay on the same side of the waterfall (don't cross the bridge) and go down. You will get to the Cafe and then 300 meters further there's a bus stop "Aftersteg/Wasserfall.Todtnau" where you can take the bus 7215 to Kirchzarten and then bus 915 back to Freiburg.
I know it sounds complicated but it's really not that hard. The hardest thing was to get to the bus stop after seeing the waterfall- simply use Google Maps as I did.
You can always check the bus and train connections from/to Freiburg on www.db.de