Guided walks led by an armed ranger, focused on tracks, smaller wildlife and plants rather than the Big Five sightings a jeep gives you.
Walking safaris are not permitted inside the Masai Mara National Reserve itself; they run in the private conservancies that border it, such as Naboisho or Mara North, where a licensed guide and an armed ranger accompany small groups. Expect a slower pace geared toward reading tracks, birdlife and plant life rather than close-up big cat sightings, which makes it a good add-on to a drive-based safari rather than a replacement for one.