East coast is the place to dive in Malaysia (unless you want to spend, in which case get out to Sipadan). Tioman can be reached by taking a bus to Mersing from Puduraya bus station, for about RM30 (one way). We go there for weekends from KL often. Then get a boat to Tioman (RM45 one way; last one leaves about 4pm); should be all pretty easy, Mersing isn't big. Tioman is duty free (hurrah!), and accommodation is generally reasonable. We go to Tekek beach and stay at Swiss Cottage (RM68-88/night). The place is OK (would be great if they were painted), but the dive shop there (Tioman Dive Centre, www.tioman-dive-centre.com) is v good. Shahrul, the DM there has logged about 4k dives, while the expats running the place are v sensible.
You can get to the Perhentian Islands and Redang by flying up to Kuala Terranganu (actually slightly quicker to fly into Kota Bharu for Perhentian) on AirAsia. Good planes, cheap, no issues, we fly them all the time. Redang is more package orientated, but both also have v good diving.
Langkawi is nice, but West coast is going into wet season, so expect weather to decline. Even though they don't have full on monsoon there (also duty free though...

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Cameroon highlands is nice for a few days, escape the humidity! Tea plantations, jungle walks and a mountain (well big hill really) with nice views.
Good day trips include the elephant sanctuary at Kuala Gandah, but avoid weekends, can get v busy. Don't know how much, as we live here and drive, would be interested to know though...
Dodge Genting Highlands...big blot on a hillside with a casino.