For sightseeing... I would consider the following:
- Puerto Madero & Plaza de Mayo Area
- La Boca & Caminito
- Walk around parks of Palermo - Rosegardens
- Recoleta Cementery
Are you into Museums...? If so, dont miss:
- Museo de Arte Decorativo
- Museo Malba
- Palais de Glace
- Centro Cultural Recoleta