I have been to Mauritius few times, the last time was about 2 months ago, and probably I will be back for another visit during December.
During my last visit we decided not to rent a car, so we mainly used taxis.
Here are the prices we paid, which I believe are quite good:
Airport taxi: 35 Euro per way
Full day taxi to travel the island (from 09:00 until 17:00): 60 Euro
Taxi from Grand Baie to Casela Park and back: 42 Euro
Taxi from Grand Baie to Black river and back: 40 Euro
Taxi from Flic en Flac to Blue Bay and back: 32 Euro
As for the tours, we ordered some tours and excursions through this website:
I know they are quite big in Mauritius, their prices were good and we were very pleased with all the tours and activities ordered. Check their website - you can find all the prices online.
Other big agencies I heard of are:
White sand tour
Enjoy your time in Mauritius