Mussulo Island is just outside Luanda, separated (mostly) from the mainland by the Baie de Mussulo. While actually a peninsula, connected to the mainland by a narrow sandy spit, it is only accessible by boat. It's a popular escape from the Luandan chaos — clean paradisaical beaches with decent tourist facilities. Because it is popular with wealthy Luandans and expats, the restaurants, bars, and accommodations are expensive.
Many boats from different locations in Luanda depart daily, with more traffic on weekends. More expensive options are faster, more reliable, and safer.
Nothing to see here other than the beaches! The Atlantic side is not very safe for swimming, so virtually everything is bay-facing.
Because this is one of the most developed vacation spots in the country, there are plenty of water sports options: diving, snorkeling, windsurfing, water skiing.