I have a 16 hour transit in Nairobi. Is it possible to organise a day trip to the Giraffe Centre, Elephant Orphanage etc at the airport. Are there operators in the arrivals area. Or is it better just to negotiate a price with a taxi driver. I don't want to prebook as, if I'm too tired, I might just get a day room somewhere.

Hi,my name is Tim and I am in Kenya, yes a trip to the girrafe centre, and elephant orphanage would be ideal but for the elephant orphanage is is between 11am to 12 pm, if you are not too tired it would only cost you $ 50 but then you pay for the entrance fees at giraffe centre and the donation at elephant orphanage, you could have lunch at the carnivore or Java,
If in need of help just give a shout here, tim

Once at the airport, there are plenty of taxi drivers around. Just take one and, after bargaining on the price, go with him/her for the day.
Elephant orphanage is indeed open between 11 am and 12 only. Giraffe center is open all day.
Carnivore restaurant is a little too touristy for my taste. Would advise you to go and have lunch at Talisman restaurant in Karen neighborhood instead. Great garden, nice food, beautiful decoration and the expatriate crowd instead of the touristy crowd you'll find in Carnivore. Karen Blixen's house is not far and can be visited as well.
If you're too tired, then there are sleeping rooms at Nairobi airport, down by gate 4 (I think). You also might get a free meal in the transit restaurant- ask at the transit desk. I had an 8 hour stop there and got a meal, and asked when I had a 5 hour stop and was told it was anything over 6 hours. That might only be Kenya airways though.
Thanks, I've used the transit lounge and restaurant before. That and the sleeping rooms (if I can find them and they aren't too expensive) are my back up plan. Seems a shame to waste a day though and, although I've been to Nairobi a few times, I've never managed to get to the Giraffe Centre or Elephant Orphange. Carnivore sounds like a group place and way too touristy for me and I will be on my own this time.
I guess it rather depends which way you're transiting. If you've just come off a short flight from somewhere else in Africa and then are flying long haul the next day then you might well be up for a day out in Nairobi. On the other hand, if you're coming in off a long flight (from Aus?) then I'd be well impressed if you have the energy for negotiating with a taxi driver and driving round sight seeing all day!!