There's been some talk about the possibility of sending an airplane to Titan, the Saturn's biggest moon and I it sounds a lot more interesting and practical than a plane on mars.
For one, Mars's atmosphere is very thin, making flying in there very hard. You're going to need big wings to stay in the air and controlling it is going to be delicate. Titan, on the other hand, has a very thick and dense atmosphere, allowing for a lot smaller craft and easier manouverability. That also lowers the power requirements for the propulsion, so it could be quite feasibly be powered by a ASRG giving it a flight time of years.
There are a lot of other very good points, too, but instead of writing about them myself, I'll just post a link to a cool blog that explains most of it quite well: http://futureplanets.blogspot.com/2010/06/aviatr-titan-plane-details.html
Here's a very informative presentation about it, too http://vimeo.com/11432536