Sharing some of the useful information that maybe helpful to 1st time visitor.

Transportation:

Getting to KK Town. Taking a taxi from the airport to KK Town will cost about RM 30 dependent on the zone that your hotel is located. There is a Taxi counter, where you can make payment for the taxi.

Getting to Mount Kinabalu. If you plan to visit Kinabalu Park but not climbing up to the summit. there are two choices - organised tour or car rental. Local bus from the Inanam Bus station is time consuming but cheap (more suitable for backpackers and adventure travellers). Unless you arrive early in the morning, book a car at the airport for delivery to your hotel late evening or early the next morning.  Use taxi for visits to things around KK for your first day.  For ideas on things to do with a car in KK check here.  Organised tour to Mount Kinabalu involves 6h in a coach and 2h in the park.  Driving is easy, roads are good and you can stay cheaply at some of the great hotels in Kundasang, Ranau or Poring

There are many car rental agencies at the airport or advertised at Tourist Information so you can compare the prices (Mostly local car rental agencies. Hertz is the only international car rental available at T1).

Map. You can buy a cheap, good detailed map from Sabah Tourism Office for RM2, if you are planning to drive around and outside KK Town.

Things to do  check here for lots of organised tours or many things to do on your own 

Food:

Local food. You can have a taste of good local Chinese food along Jalan Gaya or in the Wisma Merdeka food court whcih has a superb view of the islands.. Please try Laksa, Beef Noodle, Claypot rice, Bak Kut Teh.

Seafood.  You can visit TwinSky Seafood Restaurant. Jalan Kampung Air | Sedco Square, Kota Kinabalu, Malaysia

There are three other seafood restaurant at the same area, so you can look around to compare prices if you like to.  Be wary of live fish displayed in tanks, much live fish is caught with cyanide.  The redder the fish the more likely it has been caught with cyanide - red is lucky in Chinese culture.  Seabass, Tilapia, Ikan Putih, prawns and sotong (squid) are safe, tasty and reasonably priced.  Be wary prices are per 100g. 

Shopping:

Biggest Shopping Mall at KK is 1Borneo 8km from downtown. You can take a shuttle from some of the hotels or outside Warisan Square.  Wisma Merdeka, Warisan square and Centre point 4th floor are the best shopping centres in KK.

Local handicraft, food, snacks. You will be able to visit the handcraft market daytime on the waterfront to the left behind the post office.  Good night market in Kampung ayer - middle of downtown or Gaya st Market sundays only.

Islands (Tunku Abdul Rahman Marine Park):

Mamutik island. a quieter, relaxing beach. Great for snorkelling. 2nd choice. Manukan Island, a more commercialised resort style.

You can get to these islands quite easily by gettin to Jesselton Jetty.  get you tickets to the islands from 1-3 islands in a day. But it will better to just select 1 or 2 islands per day. You can choose to stay on the islands too, by camping or staying at the chalet or resorts

Sunset view:

You will get to see nice sunset along the waterfront about 6.30pm, of course dependent on the weather.

Have a nice trip.