You're right not to drive from Cape Town.
It's a long drive up the West & Northern Cape provinces with relatively not a lot to do or see.

Windhoek used to be a little bit of Germany in the desert. It resembles something more like Telford these days.

There are amazing places in Namibia but I would say most are a long drive from Windhoek.
Thanks to the Chinese, nost of the main roads in the country are now tarmacked.
Driving is a joy as the roads are generally empty.
As in all Southern African countries, watch out for the coppers who like nothing better than pulling over a tourist to negotiate your speeding fine (nothing too heavy).

Fish River Canyon is stunning but I would only suggest going there if you had driven up from Cape Town.
I would say the same for the abandoned mining town of Kolmanskop.

Going west, you could take a several day tour round trip crossing the Naukluft taking in the astonishing dunes at Sossusvlei. Head north to Swakupmund on the coast and then visit the southern reaches of the Skeleton Coast before heading back to Windhoek. The Naukluft is an astonishing place. It really is an alien landscape in parts.

North of Windhoek, Etosha is the jewel in Namibia's crown (book online in advance). If you're into safari, it's the place to go. It's got a great network of dirt roads & you're pretty much guaranteed to see everything including incredibly rare black rhinos. Our Autumn is the best time to go.

So many road trips but they're probably the two I would take from there.
The drives are generally long, bu as I say, driving on pretty much empty roads is a joy.

Some amazing places to stay (try a game farm) & because of the currency being weak, it's a great place to upgrade & eat out.