I would advise to start with planning how many days you have for your trip (without international flights). Based on that, decide if you want to go only on a safari or to spare a few days and combine your safari with some beach R&R - my personal advise in this case would be to leave the beach to the end of your trip. Also, depending on your arrival time, you can plan a couple of things in Nairobi, spend one night there and be on your way on your safari. I do not think that you need more than one night to rest and regroup unless you have some particular plans for Nairobi.
Once these issues are out of the way, plan your trip so that you stay min 2-3 days in one location, otherwise you will be in a state of permanent transfer between various properties. This will give you the idea of the number of parks you can visit within the days you have for your trip. From here think about what you want to see - this will determine the parks to visit. The main parks/conservation areas are basically Samburu, Laikipia, Mt Kenya region, Lakes region (Nakuru, Bogoria, Baringo, Naivasha), Masai Mara, Amboseli, Tsavo East & Tsavo West. There are other amazing places but it is impossible to list them all! Read up on those parks and see what is appealing to you.
Masai Mara (main reserve or any conservancies around) is always advisable to visit whether it is the first or the twenty fist trip to Kenya! It never fails to impress and is known for cats, Big Five, elephants, etc. So definitely plan Masai Mara, I would give min 3, preferably 4 nights.
It is worth remembering that Kenya is a fairly large country, distances are big, roads are not too good, so it is advisable to assess distances/transfer times between the parks unless your budget would allow you to travel by air. In my view air transfers are not much more expensive than road, much more comfortable, saves loads of time and makes the whole journey more pleasant and palatable. Most lodges/camps will offer an option to include game drives in their prices.
Not every travel company/tour operator and lodge/camp/hotel will be featuring in various forums or ratings, so if you do not see them it does not mean that they are bad or don't exist! Check Kenya operators on KATO website, there are many Africa/Kenya specialists overseas (US, UK, Europe, South Africa). Choose 3-5 and send them an email outlining what you want to see, the number of days and your approximate budget. Once you hear from them you can then compare what they offer, prices, check the properties they offer on internet by going to their sites, and then decide which company you are most comfortable and happy with based on your communication, how professional, helpful and flexible they are, etc.
Once you make all the hard choices :) come to Kenya and enjoy it!
Tijara Beach Hotel