Dja Faunal Reserve, the in the south-eastern part of Cameroon, is a UNESCO World Heritage Site.
There are many different animals, including monkeys, in the reserve. However, unless you are lucky you won't see them.
You are required to pay a park fee, hire a nature guard (he or she will have a gun), and probably a guide as well. Total should be around 10000 to 15000 CFA for all three.
Pygmy villages, you could pay to see a traditional dance.
There isn't much to buy, or many options.
Best to take your own food if picky. Some of the locals may cook for you if you ask (~3000 CFA).
It's best to go with a guide who knows the area and can advise on all the above. Contact a tour company (e.g. in Yaoundé).