We drove Townsville to Sydney (looooong drive ) with a 16 month old and have done Brisbane to Townsville several times (about 16 hours) with two kids at different ages, and several other driving trips. We also recently drove throughout Italy with them both, aged 13 months & 3 (almost). I have lots of experience in this area
We have tried driving throughout both the night and day in Aus, but we prefer day. You have to make sure that you take turns driving/sleeping if you do a night trip, as it can be very dangerous otherwise. I don't sleep well in the car, so I did the first leg while I was alert and then DH did the second leg (we drove Brisbane to Townsville overnight...left 6pm and arrived midday). The kids slept majority of the journey and it was great...however, both of my children sleep well in the car, we wouldn't have tried it otherwise. A negative was that the road is FULL of trucks at night, so we were constantly being tail-gated by crazy semi-trailer drivers and at one point were almost driven off of the road. For that reason alone, we won't drive through the night again.
For trips during the day, you need plenty of stops and PLENTY of activities. We use a combination of books, new toys, portable DVD player, iPad and car games (things like "Spot the..." & "What sound does a ______ make"?). We also have lots of children's CD's and regular CD's. You can also get some crafty items from the craft/art store which are easy and not very messy, so great for car trips - DS loved the blank cardboard masks that I bought, we drew animal masks and monster masks with crayons and then held them up to our faces and made the associated noise. He might be a little young for that, but you could do it and let him hold it up. Other toys that were a big hit - doodle drawing and the crayola wonder colouring books (the pens only draw in the book it comes with, but can't draw on other surfaces).