Hello Jane and Julian,
If you are great fans of nature, Mauritius is the place to be
The Botanical Garden of Pamplemousses is great! It is one of the most famous botanical gardens of the planet! You can spend a whole day there, admiring plants from all over the world and still you haven't seen everything...
You also have to visit the Black River Gorges National Park! You can do excellent hiking trips there, which will lead you through a wild "jungle" with waterfalls and rivers. And if you are lucky you might even meet some monkeys!
There are several other places worth a visit: Domaine du Chasseur, Ile aux Aigrettes, Chamarel and loads more.
You can do your trips individually: some places are easily to be reached by bus, for others a rented car might be more suitable. But of course there are also tour operators which offer organized tours...
Hope this helps.
Richard