There's plenty to do in Cape Town after dark. The city's nightlife is concentrated in a number of areas, so you can explore a different one each night or move from one hub to another. After all, the city is small enough to get around in quite easily. That said, however, walking from one area to another isn't advisable, as there are some parts of the city that are completely deserted and unsafe. Women, in pairs or singly, and couples should not walk alone.
One of the best—and safest—places to start is the Waterfront, where you can choose from movies, restaurants, bars, and pubs and walk between them quite happily, as there are plenty of security guards and other people walking around. The top end of Long Street is another good nightlife area. Here you'll find a couple of blocks of bars, restaurants, and backpacker lodges that are open late. And the area bounded by Loop, Long, Wale, and Orange streets is the best place to get a feeling for Cape Town's always-changing nightclub scene, but ask around for the latest on the current flavor of the month.
De Waterkant is very busy at night; if you're in the area, you can also take in the Green Point strip, where restaurants and bars open out onto the streets. On weekends these bars are packed, and you'll get a good idea of how Capetonians let down their hair. Heritage Square, in the city center, is another vibey place to spend an evening. Here you'll find an ever-changing mix of bars and restaurants. Mouille Point's Platinum Mile is an excellent place for evening cocktails. The views over the Atlantic and onto Robben Island are breathtaking, and this is where the ultracool set hangs out after a hard day at the beach or gym. Be prepared to line up to get into places, especially on a Friday night.