Hi Mustang Birder,
As per your request, allow me to kindly let you know that it is possible to do game drive in Aberdares!!! the only thing that is there, is that as a general practice most Kenyan tour operators they do not do game drives in this park. but should you be interested it is possible to do so. In fact Aberdares National Park is one of my favorite park if you wish to see Leopard!!! most times you get on the tracks walking. Again as Culture clients are normally dropped off at Nyeri town and then have lunch at one of the 2 hotels that have lodges in the Park and later on are transferred to the hotel by a large bus as a group, Should you then wish to do any sort of birding there it is possible to arrange a 4 x 4 car and then drive around the park, Treetops is not the best option for a birder as there are no near bushes around the hotel and you cannot be allowed to walk at the hotel, apart from the fact that this hotel was associated with Queen Elizabeth staying here in her early ages before being ascent to the status, I would recommend The Ark for a birder!!!
Birds you need to keep an eye on are Hunter's Cisticola, Aberdares Cisticola if you get to the moorland area, Jackson's and scaly francolin, Moorland Chat,Golden winged Sunbird, Abbot and Sharpe's starling, Waller starling, African Crowned eagle very common, Georgous or Four Coloured Bush-shrike.
Scarlet Tuffted ,Taccaze and malachite sunbird, Africa Green Ibis if you are lucky enough!! Ayres's Hawk eagle, Giant forest hog is another animal that you would see there.
the climate is cool during the day and can be cold at night!! being a mount habitat you must expect rains anytime and vegetation is green forest and bushes.
Hope this will help a little bit.
Regards,