I have to say I'd go with Naomi's list - it's the usual way such requirements are handled in Hollywood
As already mentioned, 6 years of physical incarceration is a LONG time and it's hard to think of any crime deserving such a sentence that would be considered 'harmless'. Even 'white collar' crime such as fraud usually hurts someone down the line and a 10 year sentence would reflect that hurt.
If your character must have committed a crime (i.e., actually done it) then I'd probably go for 'causing death by dangerous driving'. While this is usually associated with the bad and irresponsible it also applies to more 'innocent' accidents such as those caused by being unfit to drive or through distraction. The best recent example is Gary Hart, the man responsible for the Selby train crash in February 2001 in which 10 people died. He fell asleep at the wheel after staying up all night on the Net and drove down an embankment onto the tracks.
The maximum sentence for death by dangerous driving is 10 years, which is just right for a 6 year served term, although note that Hart only got 5, despite killing 10 people.
Jon