Kuantan is the largest city of the East Coast of Peninsular Malaysia with a population of about half a million. It is also the capital of the state of Pahang.
Kuantan is located about halfway between Singapore (to the south) and Kota Bahru (to the north). The city is situated at the coast of the South China Sea near the mouth of the Kuantan River. The city is not a major tourist destination, rather a stopover point for island destinations (Tioman, Redang, Perhentians, etc.) along the East Coast. From Kuantan you can also also reach the popular beach in Cherating in less than an hour with the local Bus.
Johor Bahru is the state capital of the state of Johor and the second largest metropolitan area in Malaysia. Most tourists don't stop here and just pass through the city on their way to or out of Singapore. JB is a major local transportation hub however. Since it is just across the causeway from Singapore, some travellers use it as a hub to visit Singapore from here (lower accommodation prices) or to board the train, bus or airplane.
Ipoh is the apital of the state of Perak and the fourth largest city of peninsular Malaysia with a population of 760,000 (Metro area: 1.4 Mil). It is scenically located amoung karst limestone hills in the Kinta Valley, about 2 1/2 hours north of Kuala Lumpur. The city is somehow forgotten by most travellers in Malaysia, however it has a lot to offer with many colonial buildings and shophouses (it used to be a very rich city around the turn of the 19th century due to tin mining) and the nature (hills, caves) around. Among Malaysians it is known for good food like the popular Ipoh Chicken Rice.
Langkawi is an archipelago of 99 islands in the Andaman Sea, some 30 km off the mainland coast of northwestern Malaysia. The islands are a part of Malaysia’s Kedah state, but are adjacent to the Thai border. By far the largest of the islands is the eponymous Pulau Langkawi with a population of about 45,000. The island is a declared duty-free zone.
The peak season is from December to March when moonson at the eastcoast shuts down most of the Islands there (Perhentian etc.). This makes Langkawi a good island alternative during that time period. Generally though, Langkawi can be visited all year around.