Internal flights in Cameroon are operated by Camair-Co and connect Douala and Yaoundé to Maroua and Garoua. The hop between Yaoundé and Douala (45 minutes) costs around CFA45,000 one way.
The north and east of Cameroon were inaccessible due to security issues at time of writing, but when operational flights from Douala or Yaoundé to Maroua or Garoua (around 1½ hours) cost around CFA125,000 one way.
The northwest grassland region of Cameroon is the best option for cycling, as altitudes are higher, the heat less oppressive, and there is less traffic. You see few Cameroonian cyclists, and will be the object of much attention. You won't be able to access dedicated facilities: local garages will be the best best for repairs.
It is possible to take river trips around Kribi with a small tour operator; these are in motorized canoes.
Agences de voyages (agency buses, running from depots also called agences) run along all major and many minor routes in Cameroon. Prices are low and fixed, and on some bus lines you can even reserve a seat. From Yaoundé to Douala it costs anywhere between CFA3000 and CFA6000, depending on the class of bus you take: so-called VIP services have air-conditioning and aren't quite so cramped. However, some drivers are extremely reckless, and bus accidents occur all too frequently.
Taxis-brousses (bush taxis, which are shared private vehicles) are also popular, especially to some more remote destinations.
Note that, while they might be called agences, gares routières (bus stations) and sometimes motor parks, these are really glorified car parks for buses, that operate all day and far into the night.
Car & Motorcycle
Driving in Cameroon is feasible, with mainly decent roads and little police harassment, though the driving all around you is hair-raising. You can hire cars in all large towns, but there's more choice in Douala and Yaoundé. Car hire is very expensive, however, partly because you'll need a 4WD for most itineraries: this becomes essential in the rainy season. In Douala, try Avis in the Hotel Akwa Palace.
A better option is to hire a car and an experienced local driver, who can negotiate road blocks and potholes on your behalf. Ask at your hotel for suggestions, and expect to pay around CFA100,000 for a full day, including petrol.
Cameroon's rail system (Camrail) operates three main lines: Yaoundé to N'Gaoundéré; Yaoundé to Douala; and Douala to Kumba. In 2016 a tragic derailment between Douala and Yaoundé killed 70 passengers and injured many more. Check locally about the current state of train safety.