just got back from nairobi and a brief visit to Mombasa. Serena is agrat hotel and you'll have no problems at all.
you may have alread organised transfers with serena but if not there are taxis avaliable. After you have picked up you luggage and just about to go through the doors where evereyone will be waiting...just before on your right you can order a taxi there. its a consortium of a couple of resepctable taxi firms and will give you the price there and then... the cost to serena shoule be 1200ksh to 1500 ksh tops... if you miss this booking lfacility look for a Jatco taxi as these are reputable.
once at the serena you just have to look as if you need a taxi and they will waive one over for you.
try dinner at carnivore or the moonflower or lord errol (Lord errol is a bit more pricey but superb, moonflower has a jazz band on saturday evening - nice too, carnivore if you love meat!!!..its a great place)
the giraffe manor in morning and karen blixen musem and suedo house coffee gardens for lunch - (on sunday afternoon there is a jazz band on outside..very nice)
safari walk is nice too...for a couple of hours
there is alot in nairobi...people often miss it...but i love it there
have a great time!