Roath Park is Cardiff's best public space: a Victorian park centred on a 30-acre boating lake with formal rose gardens at one end and the Scott Memorial lighthouse, erected to commemorate Captain Scott's last Antarctic expedition, at the other. The lighthouse is genuinely strange and moving in a park setting; the Terra Nova sailed from Cardiff.
The rose garden is best in June and September. The lake has rowing boats and pedalos available in season. The park is free, open all year, and entirely worth the 30-minute walk from the city centre through Roath's Victorian terraces.