There is no itinerary here in the usual sense, and that is deliberate. The day after a marathon that finishes at Parque Harmonia rewards staying close to the water rather than committing to a schedule.
The Mercado Público
Porto Alegre's central market sits in Centro Histórico, a short taxi ride from Parque Harmonia. It is an easy way to eat and look around without a full sightseeing day attached to it. Avoid peak lunchtime if a calm stomach and quiet legs matter more than a lively crowd.
The Guaíba Waterfront
The redeveloped waterfront running past Parque Harmonia and toward the old port warehouses is flat, scenic, and forgiving on legs that ran 42.195 km the day before. Sunset over the Guaíba is the reason to save some of the day for this rather than spending it all indoors.
Usina do Gasômetro
The former power station turned cultural venue sits on the waterfront and works as a stop whether or not it happens to be open that day. The building itself is a bonus; the view over the water from outside it is the real draw.
Getting Back
Everything here is walkable from Parque Harmonia or a short taxi away from central Porto Alegre. No return journey to plan, no transfers, nothing that asks more of the legs than they can give the day after a marathon.