I am not sure about hot air balloon, but I can tell you from my own experience that I traveled to Mauritius a month ago. I had a great time.
I ordered few attractions and tours online before arriving to Mauritius. I found many activities on this website: mauritiusattractions.com . The prices were good, service was very professional and we enjoyed all 3 trips we ordered through this website (sea kayaking, tour to the south, and hiking tour).
Check it,write them, maybe they can help you arrange the hot balloon trip.
Good luck, I hope this helps. Wishing you a great vacation in Mauritius.
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