I actually just came back from Mauritius and I had a great time! I can only recommend from my own experience. I ordered three excursions from a website called Mauritius Attractions few weeks before I arrived to Mauritius. I think you should check this website, they have a very big selection of excursions and tours, their service was very professional and the prices are very good (better then in other tour agencies we found while in Mauritius). In anyway – don't book any excursions through your hotel – they really charge double prices.
I hope this tip helps. Bye.
Updates
Is this your business?
P.S I just used this website to order a diving course as a present for my husband - the price was the cheapest I found, I just hope my hubby will enjoy his surprise present.