There are a few underground parking garages in the city center which aren't overly expensive if you do not park there too long, just check the height clearance, although it should not be problematic with most vans.
There is an underground parking garage outside the Market Hall near the river and another that services patrons of the Congress Center, also near the River. There should be signs for both if you head toward the river. The second garage is near the opera house and the new Haus der Musik. (House of Music).