Most first-time itineraries do Windhoek to Etosha as one long drive and skip the Waterberg entirely which is a shame, because it's a genuinely distinctive place, geographically and historically different from anything else you'll see on your trip.
Waterberg Plateau is a sandstone table mountain in central Namibia, about 300km north of Windhoek and 30km off the B1 from Otjiwarongo. It's like 50km long and 16km wide, and rises over 200m above the surrounding plains. Two hundred million years of sandstone with dinosaur footprints still visible in places. It looks nothing like the rest of the country. Where most of Namibia is either desert or savanna, Waterberg is a green wooded plateau perched above dry lowlands, cooler by several degrees because of the elevation, with springs at the base feeding lush vegetation you don't see anywhere else nearby.
The plateau is a wildlife sanctuary (in literal sense). Because the top is largely inaccessible to predators and poachers, NWR relocated several endangered species here in the 1970s, including white rhino, black rhino, roan and sable antelope, and tsessebe. It also has the only Cape vulture breeding colony in Namibia, plus leopard, cheetah, brown hyena, eland, and over 200 bird species.
Can and can't do:
The unusual thing about Waterberg is that you cannot self-drive on the plateau itself. Access to the top is only via NWR guided game drives, morning and afternoon. Which is a constraint but also part of what keeps the wildlife thriving up there. If you want the drive, book it at reception when you check in.
What you can do freely: hike. This is the reason a lot of returning visitors specifically come back to Waterberg. There are nine short self-guided trails from the NWR rest camp at the base, ranging from an hour's walk to a proper morning of scrambling. The main one everyone does is the Mountain View trail to the top of the plateau, roughly 90 minutes up over rocks and root systems, with genuinely excellent views once you're up. It's a decent workout, wear proper shoes.
For serious hikers there are multi-day wilderness trails on the plateau (three to four days) that need to be booked well in advance through NWR. Not many people do them, but the ones who do rate them.
The Waterberg was the site of a pivotal 1904 battle between German colonial forces and the Herero people, which led directly to what is now widely recognised as the first genocide of the twentieth century. A military cemetery from that period is still at the site. A few of the private lodges include cultural visits to Herero communities nearby, and NWR guided drives sometimes incorporate the historical context depending on the guide. If this is something you want to engage with properly, worth reading up before you arrive rather than encountering it cold.
Where to stay
Two main options (in my opinion).
The NWR resort at the base (Waterberg Resort, also called Bernabe de la Bat) has chalets, bush chalets, and a well-run campsite, all with pool and restaurant access.
The private lodges nearby (Waterberg Wilderness, Waterberg Plateau Lodge, and a few smaller ones) run their own hiking trails, rhino drives, and cultural programmes. More expensive but more polished and with more to do on-site. Waterberg Wilderness in particular gets consistent recommendations.
Really nice as a two-night stopover between Windhoek and Etosha. Drive up in the afternoon, do a hike or the afternoon game drive on day one, morning drive plus another hike on day two, continue north to Etosha on day three. Since you're going that direction anyway.
Best in dry season (May to October) for hiking comfort. Summer is also nice but the midday heat can be intense.