Keetham Lake, officially Sur Sarovar Bird Sanctuary, is the easiest nature break from Agra's Mughal circuit. The lake and surrounding wetland attract resident and migratory birds, especially in the cooler months, and the slower pace is a useful antidote to the Taj Mahal crowds.
This is a half-day excursion rather than a must for every visitor. It suits birdwatchers, families, or anyone staying long enough in Agra to want more than marble, sandstone, and traffic.