It should be possible to take a canoa from Cacheu to Sao Domingos, not on Sundays however, unless you pay for the whole canoa yourself.
Tarrafe natural park can be visited by canoa. Regural (non tourist) canoa trips to Sao Domingos leave everyday around 10am (may vary with sea level) from the harbour (less than 3 euros as in 2008). It is possible to have lunch at Sao Domingos and return in the same day. If you want a customized trip talk directly with the fishermen.
"Tabancas", small villages around Cacheu, can be visited with local guides.
Visit the cultural centre "Casa do Capitao Mor" created in collaboration with "Associacao de Cooperacao com a Guinea-Bissau" (Portugal)
The catholic mission sells local fruit jams to fund medical and health care activities among children.
The nightclub "Butu de Cacheu" is the only place to drink, dance and listen to african music.
Alternatively, take a car back to Canchungo or Bissau and spend the night there, there are better hotels and restaurants in Canchungo than Cacheu.