Non argueably one of world's wonders. Tropical forests, cirques, volcano, steep mountains, canyoons, watarfalls, everything can be found there.
Reunion National Park covers relatively big area almost entire interior (except those around villages), easy accesible by foot (marked trails) or bus. For those with limited time the best starting point to see the cirques is either Cilaos or Salazie (both accessible by bus).
For visiting the volcano (Piton de la Fournaise) it is better to have your own / rent car good road from Bourg-Murat up to Pas de Bellecombe (free parking). Otherwise it is a half day walk up to Gite de volcano. Piton de la Fournaise resembles Teide of Tenerife but at least the crater area is much, much bigger.