Trilha da Costa da Lagoa is one of Florianopolis' classic walks: a green route along the edge of Lagoa da Conceição, linking small waterside communities, forest, little piers, and restaurants that face the lagoon. It feels much more removed from the city than the distance on the map suggests.
Most travellers walk one direction and return by boat, which is the nicest way to make the outing feel complete. Wear shoes you do not mind getting muddy after rain, carry water, and leave time for a slow meal by the lagoon before heading back.