Well, fun.... I am not sure... or yes it will be fun to be at your parent's house and the mean to get there is the car, so yes, there is some fun involved.
The thing you will want to avoid is being carsick.
I have 3 kids , well, all taller than me now, and DS2 was always carsick (and plane sick) when young. Always sick. Always.
What I have learnt over the years is :
- no books
- no games on iPad
- no drawing or any activity with head down
- movie with head straight and not on lap
- leave window open even if AC is on (which you can't do on plane!)
To entertain with the head up, I always found the reusable stickers for the car window to be great. Something like this www.amazon.com.au/Reusable-Stickers-Toddlers-Incredible-Clings/dp/B076XTYZX1?tag=mumsnetforu03-21
and be careful with too much snacking and favour savoury over sweet. Milk is the worst to cause vomit and also to clean, and if you miss a spot, two days later, the car will smell so badly. Lollies are to be avoided. So no matter how tempting it might be, to just snack to kill time and boredom, the less food the better . Avoid fried food at rest areas (no chips)
Another thing is don't start fussing over him unless he needs it. So don't ask how he is doing, if he is hungry , thirsty (just hand his water bottle from time to time)_ , if he want a toy/a movie/ a unicorn or whatever, if he want to sing a song, don't start singing a song or playing with puppets. . Some children are perfect travel companions, I sometime forgot I had my DD in the car. The boys would be bickering, moaning and she would just sit happily.
To keep the water bottle frozen as long as possible, wrap them in a tea towel, they will de-freeze a bit, but keep the centre a big block of ice, to which you can add the warm water from other bottles.