Port-Cros is a bit smaller than Porquerolles, but it's more "mountainous" in nature and even more thickly forested. The deep-harbored little port is pretty, and guarded by ancient forts above. The walks to see nearby sites are fairly short, and there are a few beaches.
For a short visit, a 15-20 minute hike from the port up to the Fort de l'Estissac will give you a great view back down across the port area. The Fort de l'Estissac is very nicely restored and open for visits (free). It also has a tiny museum with a well-presented display of local items.
No camping is allowed on the island.
No smoking is allowed beyond the village.
No drinking water is available on the island, except at the village.
The National Park, Parc National de Port-Cros, includes the main island, the smaller adjacent islands and all of the waters surrounding this area.
There's one small beach in the bay beside the village/port. A couple of other beaches are located 20-30 minutes away, in each direction. Some visitors we saw were obviously there for a day at a beach, and you can apparently hire a ride to a beach on a small boat.
Port-Cros has a very small village area, with tourist shops and cafés rather than typical village shops.
Transportation Port-Cros Island
Our Travel-Sea page lists the passenger ferry schedules.
IGN (1/25,000) #3446 ET "Le Lavandou, Parc Nat. Port-Cros, Corniche Maures"
Didier Richard (1/50,000) #25 "Maures Haut-Pays Varois"
Hiking here is great. The trails are more crowded than those of the "back country", but the terrain is interesting without being too steep, and there's always the view of the sea through the trees. One circular path near the village is marked as a nature trail (Sentier des Plantes). With the different herbs and other plants on the island, there's always a rich Mediterranean scent in the air.
The "Information" office at the port free maps of the island, but they're not very good, and don't match the markers at every path junction; a large detailed map is normally available (about 4 €), but they had run out, so we can't comment on it.
It will take most of a day to explore either the south-and-west part of the island, or the north-and-east part. The most interesting forts are the ones near the village. The Fortin de la Vigie, across the island, isn't worth the trip.
Be sure to take water with you. There's none available outside the village, and it can get hot, even with the trees and the sea breeze.