I would suggest at least two days for Genting Highland. It is a hill station situated around millions year-old rain-forest and the cool mountain air. It has offerings for both a nature lover and the cool age millennial. It has indoor and outdoor theme parks. Has the most expansive shopping mall and a casino. You can find cafes, restaurants, bars serving global cuisines.
It is a 45 mins ride from Kuala Lumpur. Once you leave the city behind you can immediately sense a drop in the temperature. The ride has lush greenery on both sides and you can't help rolling down the windows and be in awe of the place.