Took a 111 Matatu from Junction and got off at the Karen Roundabout (Langata Road). I then took a cab for 300 Shillings to the Karen Blixen Museum. It was easy to find the reception, was greeted warmly, paid my 1200 Shillings non-resident fee and I was off.
I got set up with a tour guide and he escorted me around the property and in the house. He knew his stuff and was able to answer the questions I had. I had just watched Out of Africa, so it was cool to learn more about Karen Blixen and visit her house.
Overall, it was peaceful, informative and affordable. You will probably only spend an hour there, but I found it fun and a good use of my time on my day off.