The obvious things here — the opera house, the cemetery, the Sunday fair — genuinely earn the time, and the least obvious one is the river delta an hour north by commuter train.
Teatro Colón guided tour
Fifty-minute tours of the Foyer, Gallery of Busts, Golden Hall and auditorium leave every 15 minutes from Tucumán 1171, daily 10:00–16:45 (9:30–17:15 from 1 July to 2 August). English departures are 11:00, 12:00, 13:00 and 15:00. General admission ARS 34,000; Argentine residents with ID ARS 17,000.
Cementerio de la Recoleta
Open daily 09:00–17:00, holding more than 200 figures from Argentine history including Eva Perón, with 90 vaults declared national historic monuments. Free guided tours in Spanish leave the main entrance hourly, 10:00–16:00 on weekdays and 09:00–16:00 at weekends; visitors buy entry through entradasba.buenosaires.gob.ar.
MALBA
Museum of Latin American art at Av. Figueroa Alcorta 3415 in Palermo, open Thursday to Monday 12:00–20:00 and Wednesday 11:00–20:00, closed Tuesday. Tickets are sold online through the museum's own system; it is a two-hour building rather than a whole day.
Feria de Plaza Dorrego, San Telmo
Sunday street fair of nearly 300 stalls centred on Plaza Dorrego and running along Defensa, Humberto Primo and Pasaje Giuffra. The plaza itself is old furniture, advertising signs, vinyl, soda siphons and clocks; Defensa is contemporary crafts and Pasaje Giuffra is clothing.
Tigre and the Paraná Delta
Take the Mitre line train to Tigre, then a lancha colectiva from the Estación Fluvial Domingo Faustino Sarmiento at Mitre 305 — the delta islands have no roads, so the boats are the local bus. Three companies (Interisleña, Jilguero, Delta Argentino) run fixed routes; tickets are sold at the station and are valid 30 days, and timetables shift with water levels, so confirm with the operator.