One thing to note is that when you stay in (almost?) any hotel in the Black Forest area, you get a free travel card for the whole area (bus/train etc.) called "KONUS".
Mühlenbach is included:
http://www.schwarzwald-tourismus.inf.../konus2/(az)/M
Going around by train can be more relaxing (and giving you the opportunity to do one-way hikes) - Haslach, which has a train line running, is in the next village north (and connected by bus to Mühlenbach) - alternatively, Elzach on the south has a direct train going to Freiburg. And free parking (so you wouldn't have to take the not-so-frequent bus there, then switching for a relaxing ride all the way to Freiburg)
Also, Elzach has some nice old village centre itself, and a decent choice of restaurants (Gasthaus Rössle for a bit upscale local food, Gasthaus Zur Krone for more "down to earth" food, as two examples - but of course there is more..)
The Black Forest in summer is one of my favorite areas to go. It's hardly ever too hot, and even if it's warm, there are a lot of natural lakes to take a swim in to cool down.
On your question about which route to take towards Interlaken, I assume you'll have/buy a Swiss highway sticker?
You can either drive towards Freiburg (im Breisgau), then a bit on the unlimited German Autobahn if you fancy, then on the Swiss highway via Bern - or alternatively, if you've more time (and I mean more time, it will take you several hours longer) - especially if you want to save that sticker - all the way through the Black Forest. I'd suggest going North first via Haslach, east Hausach, then all the way South on Route 33, switching to Route 500 to Triberg, Schönwald, Furtwangen, along the Titisee (you might want to skip that one, super touristic), along the Schluchsee (still on route 500) - now going on smaller roads towards Grafenhausen. Several options South now, you just want to drive through the Schwarza-Schlücht-Tal, which is as much Black Forest as it can be (no worries though, the road is in excellent condition) towards Waldshut-Tiengen (Swiss boarder)
In Switzerland, go via Brugg, Lenzburg, Birrwil along the Hallwilersee, then cross over to Beromünster (Home of the "famous" Radio Beromünster -
https://de.wikipedia.org/wiki/Landes...rom%C3%BCnster )
If you're into storks, Sempach has THE stork center of Europe (Vogelwarte Sempach) - plus, Sempach itself is a fantastic preserved old city.
http://www.vogelwarte.ch/de/voegel/v...issstorch.html
From there, go towards UNESCO Entlebuch (the city is called the same) - more information here:
http://www.myswitzerland.com/en-ch/u...entlebuch.html
You can take the Rothornstrasse, then the Panoramastrasse, all the way over the mountain pass (in summer only)
In Giswil, turn South again, along Lungernersee, up the famous Brünigpass, then down to Brienz and long the right hand side of the lake all the way to Interlaken.