I think Cassini should've used a balloon for Titan. Huygens was successful and all, but why limit yourself to only minutes of observing after going all that way? Venus would be another great candidate for dirigible observations.

For mars, I like building a very powerful (nuclear powered) rover. This should be able to operate for years and cover a lot of ground fairly quickly. It could have a reinflatable dirigible tethered to it that could have some very light weight instruments to perform visual inspection/other studies from the ground up to a few hundred feet say. Maybe this tether could be one of the first applications of a carbon nanotube tether--super light weight and possible conducting/communication layers if desired.

With the airborne survey, the rover could possibly navigate(anticipating) fairly quickly with some good routing software.