J.A.
Well, it might be inconvenient to the two adults, but maybe leaving really early in the morning or late at night so they are sleeping for most of the trip could help.
We plan on traveling to Albany with our daughter soon (4 hour drive). I intend on leaving right around her nap time (11 am) so she sleeps for half of the drive.
Always keeps your itinerary open, though. The kid may need to get out and walk more often that you anticipate. So count that into your travel time.
I hear the travel DVDs are amazing for older kids. I don't know if a 2.5 year old would be too interested....