Soyo's remoteness and limited transport options constrain day trips, but there are a few worth considering.
Across the Congo into DRC — The DRC (Democratic Republic of Congo) begins on the opposite bank of the estuary. A crossing is theoretically possible by boat, but requires valid visas for both countries and careful research into current border procedures. This is not a casual excursion; consult recent traveller reports before attempting it.
The coast south toward Cabinda — The coastline south of Soyo toward the Cabinda exclave is largely undeveloped. A 4WD vehicle opens up beaches and coastal scenery that almost nobody visits. The road quality deteriorates rapidly away from town, and you should not attempt this alone.
Mbanza Kongo — Angola's most historically significant city, the former capital of the Kingdom of Kongo, lies several hours to the southeast by rough road. The old royal city, now a UNESCO World Heritage Site, makes for a worthwhile extended trip if you have the time and a reliable vehicle. Plan for an overnight stay rather than a single day.