One of the most spectacular parks in Europe is the Bois de Boulogne, often called the "main lung" of Paris. Horse-drawn carriages traverse it, but you can also drive through. You can discover its hidden pathways, however, only by walking. You could spend days in the Bois de Boulogne and still not see everything.
Porte Dauphine is the main entrance, though you can take the Métro to Porte Maillot as well. West of Paris, the park was once a forest kept for royal hunts. It was in vogue in the late 19th century: Along avenue Foch, carriages with elegantly attired and coiffured Parisian damsels would rumble along with their foppish escorts. Nowadays, it's more likely to attract run-of-the-mill picnickers. (Be careful at night, when hookers and muggers proliferate.)
When Napoleon III gave the grounds to the city in 1852, they were developed by Baron Haussmann. Separating Lac Inférieur from Lac Supérieur is the Carrefour des Cascades (you can stroll under its waterfall). The Lower Lake contains two islands connected by a footbridge. From the east bank, you can take a boat to these idyllically situated grounds, perhaps stopping off at the cafe/restaurant on one of them.
Restaurants in the bois are numerous, elegant, and expensive. The Pré Catelan contains a deluxe restaurant of the same name (tel. 01-44-14-41-00; www.precatelanparis.com), occupying a gem of a Napoleon III-style château, and also a Shakespearean theater in a garden planted with trees mentioned in the bard's plays. Nearby is La Grande Cascade (tel. 01-45-27-33-51), once a hunting lodge for Napoleon III.
Jardin d'Acclimatation (tel. 01-40-67-90-82; www.jardindacclimatation.fr), at the northern edge of the park, is for children, with a zoo, an amusement park, and a narrow-gauge railway.
Two racetracks, the Hippodrome de Longchamp and the Hippodrome d'Auteuil, are in the park. The Grand Prix is run in June at Longchamp (the site of a medieval abbey). Fashionable Parisians always turn out for this, the women in their finest haute couture. To the north of Longchamp is the Grand Cascade, an artificial waterfall.
In the western section of the bois, the 24-hectare (59-acre) Parc de Bagatelle (tel. 01-43-28-47-63) owes its existence to a bet between the comte d'Artois (later Charles X) and Marie Antoinette, his sister-in-law. The comte wagered he could erect a small palace in less than 3 months, so he hired nearly 1,000 craftsmen (cabinetmakers, painters, Scottish landscape architect Thomas Blaikie, and others) and irritated the locals by requisitioning all shipments of stone and plaster arriving through Paris's west gates. He won his bet. If you're here in late April, it's worth visiting the Bagatelle just for the tulips. In late May, one of the finest rose collections in Europe is in full bloom.