Hi Catherine!
I think you have the right idea going to Ol Pejeta then Samburu. They are very different habitats from the Maasai Mara, and have some unique animals endemic to that area, like the Grevy's zebra and the Somali ostrich. I stayed at the Samburu Sopa lodge which was completely satisfactory. I sat out most early mornings, watching the animal movements in the bush and hoping it wasn't a lion- surprising how everything looks like a lion in the dark!!! At Ol Pejeta, I stayed at Sweetwaters...a truly lovely place but a touch too colonial for me(I don't really like to have to 'dress' for dinner!). I did see rhino, though- a privilege!
The only other suggestion I would have is to think about trying Tsavo East and West. These 2 parks are also very different habitats from the Mara, and although I spent 4 days there, this time I have planned for a week, I loved it so much! I stayed at the Ashnil Aruba Lodge, which while being somewhat tourisity, was more than compensated by the elephants at a watering hole only 50m away! I could FEEL the rumbling!!! I also stayed at Ngulia Lodge-jaw-dropping views, and challenging spotting the animals in the bush. I had some misgivings about the leopard baiting this lodge practices every evening by hanging a goat leg from some tree branches. While it was exciting to see a leopard so close, it felt a bit contrived. I realized that fully when I was lucky enough to see a leopard in a much more natural way in Samburu strolling on the banks of the river...completely different experience!
I hope you have a wonderful trip, and please let us know what you decide!