Unless you want a specific lodge or camp, you can leave booking until you get to ATBR. My advice would be to contact a couple of companies for suggestions and quotes, we have used both WT safaries and JT safaries and have been very happy with both companies. This year We went with WT safaries, 2 nights and 3 days staying at Saltlick and Sentrim in Amboseli and paid £650 for a mini bus with just the 2 of us, if we had 4 in the mini bus we would have had to pay £600 each. Amboseli is an 8 hour drive (though so worth the travelling) so a trip to Tsavo would be less and a trip to the Maasi Mara would be more. Cotts Tours and Travel have an office at the hotel and you can email them at their Nairobi office and ask them to give you a quote for a safari. Merci who is the Thomson Rep, is fantastic and will help you book any trip out but if you book a Thomson safari expect to pay more than booking locally and to have a full mini bus.
Pedro and Tracey at the Barracuda Dive school have lived in Kenya for many years now and can help you book trips of all kinds, including fishing trips and safaris, the offer really good snorkelling trips for families, it is well worth emailing them with you requests for safari prices and fishing trips.
Barracuda Dive School -divediani.com
WT safaris- www.wtsafaris.net
JT safaris -www.julius-safaris.com
Cotts travel and tours - www.cottstravel.com
Amani Tiwi Beach Resort is a wonderful hotel and you can look forward to a wonderful holiday during which you will be made to feel really welcome and special. Have a lovely time.