Douala may not be the capital (Yaoundé is) but it's Cameroon's liveliest city. The downtown market is as bouncy during the day as the clubs are at night, the architectural influence in the suburb of Bonanjo is German, and the Musée de Douala, however ramshackle, is good for background.
The only country in West Africa with a large Bantu-speaking population, Cameroon has some of the best-preserved rain forest in West Africa, as well as a sizeable number of Africa's shrinking population of pygmies, who stay well hidden around the town of Kribi. Cameroon's musicians are some of the finest in Africa, and if you get the chance to hear Manu Dibango or Moni Bile play live, don't turn it down.
The food in Cameroon is excellent. Bongo tchobi comes with a black sauce made from forest seeds, bobolo is fermented cassava (not unlike plantain) and ndolé is made from shredded leaf and melon seeds. If you don't find the ingredients in the daily market at Maroua, there are several fine restaurants that will prepare something equally tasty.
For outdoor pursuits, the camping around the Bamenda Highlands is spectacular, or you could gamble that the volcano beneath Mount Cameroon won't spurt into life while you're climbing it.