Just outside Sekenani Gate, Mara Explorers Camp is a fine budget option, with dome tents or camping up to en suite tents and cottages. The accommodation is simple by Mara standards, but the price is about right and it's a traveller-savvy and a good place to meet other travellers.
They also rent out 4WDs (with driver, KSh15,000 per day) for excursions into the park. It's signposted off the main road in from Narok, a couple of kilometres north of the reserve entrance.