I think it's a great idea - I did a similar road trip when I was a child during the school holidays and it was very enjoyable.
Do get an updated map of the highway and neighbouring towns/villages. You might be tempted to take the trunk roads but bear in mind that there will be few stops in between and accident rates are quite high along these stretches of roads.
Popular stops would be places like Mersing, KT and so forth where you can take a break, hop to some islands (although a day trip may seem "too" short for some islands) or just enjoy the local sights/food/views.
Do be careful when you're driving along the northern highway linking the east to west coast - it's notorious for thick fog/mist.
In terms of safety, the usual applies - don't leave any valuables in your car and don't put objects that may hint at any valuables (even sunglasses!!!) on display. There should be some places for you to park but worse come to worse, head off to the police station, and park there (that's what my dad did).
