There are few activities more exhilarating than a walking safari in a park that's full of lions, elephants, and other big game.
Walking safaris complement, rather than replace, regular game drives because large animals tend to be wary of people on foot and so the kind of close encounters you can get from a 4x4 aren't very likely.
But, knowing they're possible is a wonderful thrill. While walking safaris are common in southern Africa, the concept has not really caught on in east Africa. Which is perhaps why so few people are aware that walking safaris are available in the Serengeti, even though they have been on offer for many years.
You can either do a multi-day trek sleeping in tents along the way or stay at a safari camp that's allowed to lead walks in their area. Both options can be customized to your experience and expectations, so even if you aren't normally the adventurous sort, you can still give it a try.