Walk - The city center is quite small, and you can walk from one end to the other in less than 30 min. Safely crossing the street can be an issue, but most streets are at least equipped with sidewalks. You can also walk to and from the airport an hour or two each way. Even the highways outside of town usually have a walking path or a wide grassy shoulder to walk on.
Taxi - Short fares around RM10, longer trips around RM20. A whole day shouldn't cost more then RM300. It should be stressed that haggling is vital in order to get a fair price. Many taxi drivers in Malaysia are infamous for raising their fare prices, especially upon unknowing tourists.
Bus - RM1.50 from Kota Kinabalu International Airport Petagas to the city centre, while RM1 from Terminal 2 International Airport Tg.Aru to City Centre. New City bus service is cheap, clean and reliable.
Hire Car - cars can easily be hired on the spot from the hire car counters in the arrivals hall. Rates can be negotiated so compare the offers.
Motorbikes for hire -
GG Rent A Motorbike Provides scooters to semi auto's for tourist and at a price of RM20. They provide maps and plan your itinerary with you to maximise your schedule and minimise the time.