You carry your lunch into the park and eat properly outside it — Nakuru town is a working Kenyan city rather than a resort strip, and that is the point of it.
Picnic inside the park
There is nothing to buy inside the fence and you may only leave the vehicle at designated picnic sites, so lunch is whatever you carried in. Park lodges and Nakuru hotels will both make up a box if you ask the night before.
Nyama choma in Nakuru town
Goat and beef are ordered by weight rather than by portion — half a kilo per person is the usual — grilled whole, chopped at the counter and eaten by hand with ugali and kachumbari. This is the town's ordinary Saturday meal, not a tourist performance.
Lake Elmenteita Serena Camp
Half an hour away in Soysambu Conservancy, and the nearest thing to a proper meal with a view outside the park. It is a full-board camp rather than a walk-in restaurant, so ring ahead rather than turning up hoping for lunch.
Mahindi choma and the roadside braziers
Roasted maize, boiled eggs and mutura are sold from charcoal braziers at the stages and along the A104, and cost almost nothing. It is the standard thing to eat between a morning drive and a late lunch.