Namibia advice

Hi all, I am planning a trip to Nam next year. My route is fairly well mapped out. It seems that the following destinations are very popular: Ai-Ais, Fish River Canyon, Luderitz and Kolmanskop, Sossusvlei, the coastal stretch from Walvis to Henties, Spitzkoppe and Brandberg. I do not intend to go further north than that. Will be returning via Windhoek and the Trans-Kalahari. Plan is to go in April or May. Apart from these destinations, I am almost more interested in identifying gravel routes that are in their own right proper destinations, especially with regards to landscapes. Any suggestions?