Mayumba is a city in Gabon.
Bush taxis will take you to Mayumba from Tchibanga and points north. There are usually bush taxis from Tchibanga at 6am. You may have to transfer to another bush taxi along the route.
Mayumba is small and you can walk most places in town and also to the beach.
Great, totally un-touristy beaches! During certain times of the year, you can see sea turtles laying eggs and hatching, including the leather-back turtle. Walk along the beach with a flashlight at night and see literally hundreds of the giant creatures. Check dates for the laying season.
There are a few rustic guest houses in town. Your bush taxi driver should be able to point you in the right direction for the nearest "case de passage."