Kenya without a car

Which are Kenya's best national park (for wildlife primarliy) for someone without their own car?
Which parks are easy to join a tour/safari in the nearest town?
We are entering from Ethiopia and would like to visit Marsabit and Samburu. How will we go there without a car?
For Masai Mara, ETA mid July, where should we be looking to join a safari (or should we rent our own vehicle for this?)? Any specific companies, esp for the more budget minded traveller, would be very appreciated.
Sorry for the vaugeness of the questions. Reading Kenya guide at the moment and it all sounds good.
Thanks

Even if you could get to the park without a car, you couldn't enter without a car at most parks. I mean, what do you think you'd be doing once you reach the park? Walk amongst the buffaloes?

The few parks that could meet your needs:
1) Hell's Gate. You can walk/cycle in the park. Go into the canyon. Few predators though, besides the occasional hyena. There's also a private sanctuary on the left side of Lake Naivasha that you can walk around in (the one with the crater lake). And yes, I walked too close to a herd of buffalo and had to beat a hasty retreat.
2) Kisumu Impala Sanctuary. Glorified zoo. But there are impala and zebras on the walking trails. May see hippos in the morning or evening. Heard they introduced rhinos a while back.
3) Saiwa Swamp. Maybe. Haven't been there.
4) Ruma. Maybe. Haven't been there.
5) Kakamega Forest. Birds, baboons and monkeys mostly. If you're into that kind of "wildlife".

Last year, KWS still had a bus that would bring people into Nakuru NP, at least up to the Baboon Cliff Viewpoint. It's weekends only and usually only the locals use it. I don't know if they go anywhere else besides the entrance and viewpoint and they might have discontinued the service.
There used to be a service to Nairobi NP on weekends too. But I've never seen that bus anywhere. The bus is supposedly green and yellow, kinda like the Citi hoppa.

bakka,

You can hire a taxi in Marsabit town to visit Marsabit National Park. It shouldn't cost too much. The nearest gate is only 1 km from the town, and the crater viewing area another 3 km. There is a small fee to take a car inside and you will have to pay another small admission fee for your driver. That being said, I don't know the current wildlife situation there. Animals migrate in and out of this small park.

Samburu visits are usually done as part of a bigger safari organized out of Nairobi. If you are coming from Ethiopia and want to enter Samburu along the way, you may be able to organize a vehicle hire in Isiolo. All I can suggest is to ask around in the town.

You cannot rent a car and drive into Masai Mara. You have to go with an organized tour group with actual safari guides. The guides know where to find the animals, and they are armed should any big animals feel like causing a problem.

sonrisadetortuga, I have never done a game drive with an armed ranger in the vehicle, in the Mara or elsewhere! Armed rangers are for the bush walks, they wear green fatigues and they should not be confused with the "guides" who are unarmed.

Re rented cars, if you mean a two wheel drive sedan, then yes, the rangers could deem it unfit to drive in the park. But whether it is rented or not is not their concern. Plenty of people have done the Mara in rented vehicles, mostly four wheel drives.

#4 is correct. Lots of people self drive in the Mara!