Started by Hyder Ali in 1760 and extended by Tipu Sultan, Lalbagh covers about 240 acres in the south of the city. It has centuries-old trees, a lake, and the Glass House, modelled on London's Crystal Palace, which hosts the famous flower shows around Republic Day (26 January) and Independence Day (15 August).
The Lalbagh Rock, part of the Peninsular Gneiss, is among the oldest rock formations on Earth. On top of it stands one of the four Kempegowda towers.
Tip: come early in the morning, when the walkers are out and the light is best. There are gates on four sides; the West Gate is closest to the Glass House.
Further reading: Lal Bagh on Wikipedia.