Mauritius has so many great places ... I'll try to add 3 of my favourites here as well.
1. Tamarind Falls
To visit these waterfalls can be the best day of a visitors holiday. Most people I have been there with, told me this was the thing they liked most in Mauritius. Small pathes lead down to the waterfalls. The view at the falls and the huge valley are indeed breathtaking. On each of the 6 steps you can have a bath in the basin and feel like you are actor in a movie.

2. Whale and dolphin watching
If you are on this Island you should have a look at those great natural dolphin show . Every morning you can see the dolphins in the Black River area. Several Boats are going out to watch the dolphins playing in the bay of Tamarin or nearby. It's just a pleasure to see how much fun the dolphins have to show what they can do. And for those who believe the dolphins want their peace and it might be a hassle for them...If you have ever seen how quick dolphins can move away in the open sea or just dive in longer sessions, there is not much chance for you to see them. On the other hand it really seems they are enjoying people clapping their hand and watching them making air roller.
3. Beach of Belle Mare
Last but not least there is the wild and unspoilt coast of the east. For me personally Belle Mare has got the most beautiful beach in Mauritius. Up to know it is not overloaded with hotels and bungalows. In the morning while doing a walk you can see lots of fishermen like hundred years ago. A tourist should not miss to see the east coast.