As far as I know the highest main road crossing of the Equator in the world is at Timberoa. This is on the road North from Nakuru to Eldoret. It is over 9000ft (9134? or is that the highest point on the road?)
There are some nice signs for photography at Maseno (on the road from Kisumu to Busia). However, it is not a day trip from Nairobi, unless you decide to fly to Kisumu in the morning and back in the evening!
On the Kakamega - Kisumu road, the signs have disappeared many times and you have to know where the crossing is. There is an enterprising school which has put up a sign fairly recently. The old sign was wonderful, but it went long ago. I do have a photo of it somewhere, because it gave the altitude as 1434 feet - under the water of Lake Victoria!
Have a look at a map and take your pick - some have better signs than others! (There is one road north of Nakuru where you cross it twice with very interesting signs and no curio shops.)