If one is going out to Karen to see Karen Blixen's house, one should also stop y and visit the Langatta Giraffe Centre nearby, to see the Rothschilds giraffes and the nearby splendid Giraffe Manor - the latter is best done in the afternoon after all the schoolkids are gone on weekdays. Carnivore restaurant is not far from here, so a supper for true carnivores is an opportunity to experience a variety of wild game dishes.
Believe it or not, one can see quite a bit on a drive-through of the Nairobi National Park - the Roots filmed their hippo classic here (and it is where Alan was gored badly through the leg and Joan was lucky enough to wind up with a couple of holes in her dive mask from a hippo canine). Be cautious with the vervet monkeys, as they are cheeky and will steal things (especially foodstuffs) from you quite boldly.
Visit the National Museum - Joy Adamson's paintings of native costume are there, as well as the snake farm, and some good dioramas on history and natural history. In the vicinity is the Norfolk Hotel, famous or infamous, depending on your POV, for the hijinks written about in many books.
Out of town you can go to Lake Nakuru, etc.
Maasai Mara, well, it's nice, full of posh lodges and now 60 - 80% fewer game than not so long ago, unfortunately. Upland, you can spend some time up in the Aberdares and Mt. Kenya to see some upland game (it can get chilly!) and if one hasn't seen northlands game one can go on a 2 - 3 day trip up to Lake Bogoria (amazing birds, and crocs,) or Samburu (Grevy's Zebra, Reticulated Giraffe, Gerenuk and other north specialties.)
Let's Go Travel on Muthaiga Road can assist with many arrangements - even a couple of days at Lamu.