mmm lots of options.
We drove down to Lake Como last year...
avoiding french motorway tolls - via Belgium, Luxembourg (cheap petrol).
Stayed in Baden Baden, an interesting place, then early start drove the fantastic 500 (http://en.wikipedia.org/wiki/Schwarzwaldhochstra%C3%9Fe ) through the black forest. Triberg waterfalls for lunch. We chose to stay a couple of nights in Schluchsee as a base for some walking.
Then on into Switzerland, o/n in Lucerne, and then the superb drive through the Alps, the route depends on when you go through as some passes are closed until June
Check out Pistonheads forum, good resource for info on passes.
Something we would have liked to do is go across Como on the car ferry from Menaggio to Varenna, a great spot for lunch.
Didn't go over Garda, maybe next time!