The first thing you have to check out is the Volcano. Many years ago it was an active one destroying the Island, yet now it has not erupted for many many years. It is quite a spectacular site. The best way to describe it is a gigantic whole filled with dense and flourishing forestry.
Going South you will also pass the Black River Gorges National Park. What a stunning park. Go into every lookout point as you will not regret it. Amazing mountains and cliffs with breath-taking waterfalls. A warning: steer clear from the monkey, they are not friendly. What’s also nice to know it that there are multiple stand selling very cheap souvenirs in this park and it is a perfect place to get your family and friends gifts from Mauritius.
When driving up in the Mountains be on the lookout for restaurants. Most restaurants serve delicious Mauritian cuisine and the views here are amazing.
What is also a must-see attraction in this direction is the Chamarel, the seven coloured earth. This should preferably be experienced on a bright sunny day as you can`t really see the colours if it is cloudy out. Either way a great attraction to see.
If you can only fit one roadtrip into your trip I would suggest this one. It has so much to see and the sites are conveniently close to one and other.