The only flight from a non-African city is from Paris. Historically, the main entry to the city was by boat up the Chari and Logone rivers, but today this is rarely done. The city has no railway links.
A muslim city, there are many mosques to see in the city, as well as a cathedral and the presidential palace. There is also the Chad National Museum, although it has lost most of its artifacts from being looted during recent civil unrest. And do not forget the views across the river.
Go shopping in the local markets- even if only for an afternoon snack- that are scattered throughout the city. This lets you experience the everyday life and culture of Chad, and also helps contributes to the country's poor economy.
Chad is a predominantly Islamic country, so do not expect alcohol to be as widely accepted as in other countries.
N'Djamena has the only international airport in Chad. Leaving the main city may be dangerous, as high levels of civil unrest are currently being experienced in the country. Travelling to the Tibesti Mountains is not safe, either, as this is a militarily sensitive zone. As in all third world countries, stay safe and have caution in your actions.