Known as the 'green lungs of Saigon', the Can Gio biosphere reserve to the south of the city covers around 80,000 ha of the river delta and is cloaked in rich mangrove forest that is full of wildlife, with over 200 species of fauna and 52 flora. This huge mass of green so close to the centre of Ho Chi Minh City helps clean the air of the city, and protect against coastal erosion and storms coming in from the sea.
The mangrove forest in Can Gio was originally over 2000 years old, and so dense that the interior was cold, foggy and dark, and full of dangerous animals such as tigers and crocodiles. During the war the mangroves were a dense hiding place from which to launch attacks on the city, which both the Vietnamese and Americans found extremely difficult to penetrate or patrol. Sadly this lead to the Americans dropping a huge amount of chemical defoliant or dixoins on the area, significantly damaging the mangrove forest, in an attempt to maintain control.
Now the mangrove is beginning to recover and is protected by the government and international law, and there are efforts to reintroduce much of the wildlife that fled the area. It is a great place to visit from Saigon, whether as a day trip or for a longer stay - there is a resort to the south with a pool and small (though slightly muddy) beach.
I loved Can Gio, and have been back several times since - it is particularly special as when you live in Saigon it seems you must drive for hours to escape to the countryside, yet just an hour south is this beautiful, thoroughly rural nature reserve. You can make the trip by motorbike - through District 7, then south through Nha Be until you cross the Nha Be river by ferry. From there head south through Can Gio to head deeper into the mangroves. We took a boat trip to explore by water, stopping off at some nature reserves and spotting all kinds of wildlife, and I can highly recommend it.
Another option is to take a ferry to Can Gio from Bach Dang wharf in Saigon, which will take you down the rivers and canals to Can Gio - I've not done this trip but I expect its a great option for those without bikes - or to book a tour of Can Gio, either locally or online through Isango.
Its worth noting that Can Gio is the poorest district of Ho Chi Minh City, with most people making their money from fishing and catching crabs, so while you are there do try to spend some moey in local shops and food stalls, tip your guide or boat crew and if you like the area, consider staying in one of the hotels or guesthouses in the region.
I forgot my camera when we took a boat trip, so my photos are mainly of our bike ride round the north of Can Gio province - you can see some good photos of the mangroves on flickr - and of the sea shore on the other side of Can Gio at Phuong Nam Pearl Resort.