Piton de la Fournaise
The Piton de la Fournaise ("peak of the furnace") is a shield volcano; that is, a volcano that emits fluid lava as the Hawaiian volcanoes do. It has frequent eruptions.
One can hike inside the caldera of the volcano, or view it from an overflight in a tourist helicopter or airplane.
Into the caldera
You must have a car, though some group tours may be available.
The N3 route des plaines ("road of the plains") cuts a diagonal in the island, from Le Tampon to the eastern shore. A clearly signposted road forks off from it and goes in the direction of the volcano (le volcan) and the pas de Bellecombe. After a few kilometers the local road becomes a well-maintained forestry road, and in the final kilometers it becomes a well-maintained track.
Park your car at the parking lot at the Pas de Bellecombe.
It is better to arrive early in the morning, for mid-day fog will set in. Remember that the northwestern coastal roads are frequently jammed, and that the route des plaines and the volcano road are mountain roads, so it may take a while to drive there from Saint Denis.
It is impossible to climb from the seashore onto the volcano - there are no paths or trails, the volcanic rocks are difficult, fog may set in, etc., so the only access is through the Pas de Bellecombe.
However, driving on the N2 coastal road at the bottom of the volcano is very interesting. The road cuts across volcano rock. Sometimes this rock is fresh - occasionnally, erupting lava will cut across the road. The highway services then rebuild the road on top of the new rock and signpost the year of the eruption.
In the beginning of an eruption, the authorities close access to the volcano, while experts map out the situation. Then, they generally open access to certain areas. Follow the instructions. It is probably much easier with a local person.
Eruption are a major attraction. Expect traffic jams.
The only method is to hike inside the volcanic enclosure (enclos).
From the Pas de Bellecombe: there is an entrance gate into a well-marked path, which occasionnally turns into a staircase, down into the caldera.
The path is marked by white paint on rock. Stick to the path. Certain areas are off-limits due to danger.
Since the Piton is an active volcano with frequent eruptions, the maps of the volcano and paths in e.g. guidebooks may be outdated - the paths are redrawn by Forestry services according to the current situation.
See & do
The various craters. If you do not want to do a full tour, at least see the Formica leo crater at the bottom of the Pas de Bellecombe.
Eat & drink
Food and drinks are not available inside the park. Take your own and do not leave traces.
The main volcanic danger is not eruptions: these are predicted by the geophysical study services following seismic and other detectable activity. The main danger is fog and temperature differences.
The volcano is a high altitude environment (2600 m). When there are no clouds, the sun is really scorching - there are no trees or other opportunities for finding shade, and the light reverberates on the rocks. Sunscreen is then strongly advised. Yet, clouds and fog may come in very quickly; then, the temperature falls and it is impossible to find one's way if one has strayed from the marked path.
Always carry a flashlight, drinking water and layered clothing, including a rain jacket. Wear hiking shoes.
Get the weather forecast before going to the volcano, and do not hesitate to postpone the trip should bad weathered be announced - if the danger is not sufficient to convince you, just think you won't see much up there with everything cloudy and rainy.
Carry a turned off GSM phone for emergencies. Note that not all the volcano is covered by GSM.
Certain areas are marked as "off-limits" by the authorities - typically because of unstable ground. Respect them.
Just drive out.