I suppose Mombasa could have been a last minute emergency, but if Obama’s grandmother (that is, Daddy Obama’s mother) was there, there must have been a bit of advance planning. I don’t know how hard or easy it was to travel around Kenya at that time, however, so who can say?
From what I've read, not that easy. Most travel was by rail, and they were still using steam engines on narrow gauge rails. Commercial air travel in the early 60s meant flights two or three times a week. I don't know how hard overland by car, but I suspect the roads were very bad. Anyone familiar with road travel in Kenya at that time?