Far from criticizing the social life that the Ilha do Cabo offers, I'd like to mention that it seems that a lot of expats only see the clubs and restaurants on the Ilha and central Luanda....oh, and occasionally Belas Shopping. Granted, if it's your fellow expats you're looking for, that is where you'll find them.
Now I'd like to mention one or 2 places that I've experienced near and around Luanda (I've not traveled Angola extensively and only move around near the city):
Mussulo: Grab a boat-taxi and pop over to Mussulo. No need to stay over night.... just head for one of the restaurants or pack a picnic basket and enjoy some peace and quiet, just a stones throw form the chaos of the city.
Barra do Kwanza: Situated at the Kwanza river (65km south of Luanda). Beautiful, relaxing, etc. Sure, not the cheapest spot to book into, but you'd be spending just about the same amount in one of the city night clubs.
Praia do Sangano (100km south of Luanda): An absolutely gorgeous retreat where you can just kick back and relax. Camp on the beach or rent a cottage for the weekend...... you'll head back to Luanda full of energy, ready to start complaining about the woes of living in the craziness of a city.
These are only 3 alternatives to the places in the city...please feel free to comment and suggest more sites and places.