Lake Naivasha is a great place to spend some time because it is the gateway to three national parks: Lake Naivasha National Park (boat tours and Crescent Island Game Sanctuary), Hell's Gate National Park (mountain biking with the animals) and Mount Longonot National Park (scenic crater hike).
From Nairobi, you can take a matatu minibus to Naivasha. The matatus leave frequently from the city center around Map Location: -1.284842, 36.830737 and are very cheap.
If you don't want to take a matatu, you can book a 1 day tour to Lake Nakuru to see the flamingos and rhinos and then arrange for them to drop you off at a Lake Naivasha hotel after the safari. The Lake Naivasha hotels are very close to the main road back to Nairobi.
Arranging transportation in the Naivasha area is very easy and affordable. There are multiple options:
Boat - Hotels have their own boats for lake tours and pick-up and drop-off at the Crescent Island Reserve where you can walk amongst the animals with a guide or on your own. This is a fantastic experience.
Matatu minibus - These run frequently along the lake road and charge 100 KES or less per trip. They are usually packed full and make frequent stops so be prepared to squeeze in.
Safiri app / Wasili app - These are the Uber services for the Naivasha and Nakuru areas. They allow you to use cash instead of credit cards and allow you to get rides for set prices. There seems to be more cars available on the Safiri app.
Hotel Driver - The hotels usually know a driver and can arrange your trips. For example, I used Nathan the Safiri Driver when I stayed at Burch's Resort. Nathan will drive for the prices shown in the Safiri app but you can call him directly to arrange transportation at +254 724 678 510 (international SIM) or 0724 678 510 (Kenya SIM).
Taxi / Motorcycle Taxi - Be prepared to negotiate a good price when using taxis. The motorcycle taxis are usually waiting in groups and wear orange or yellow safety vests.