Kuala Lumpur

Places to visit: Pavilion mall, Suria mall, Chinatown (area around Petaling Street Market), Little India, Jalan Alor pedestrian street
Pavilion mall is one of the best places to visit in the downtown. Inside, I recommend visiting Din Tai Fung (Taiwanese chain), Sushi Zanmai (Japanese chain, make sure to order the salmon belly (Toro Salmon)), Daiso (Japanese dollar store), and the basement food court
Food places to try: Din Tai Fung (Taiwanese chain), Sushi Zanmai (Japanese chain), any Indian restaurant (specifically try gobi manchurian)
Shops to visit: Daiso, Don Don Donki, MiX Store
If you want to try fresh soy milk or buy a cheap suitcase, Petaling Street Market is the place to be
I am a big fan of these drift karts at Sungei Wang mall
The Merdeka 118 is the 2nd tallest building in the world, but the observation deck is not open as of Q1 2025. Instead, you can buy a ticket (at least 1 week in advance) for the Petronas Towers
Near Bukit Bintang you will find little Arabia, with loads of Kunafa shops, shawarma, and more
Sky dining is a popular offering near the Petronas Towers, with many restaurants in high buildings offering amazing views

Langkawi

For me, Langkawi was like a Thai island but with slightly higher prices, more amenities, and much fewer cheap backpackers. Overall, a worthwhile experience as long as you aren't one of those cheap backpackers and prefer something a bit nicer.

In addition to visiting the simplistic Cameron Valley Tea House 1 or 2 with lovely views of the tea plantations, my favorite place in Cameron Highlands is the Jim Thompson Tea Room. They have live piano music, a scrumptuous British afternoon tea, and a nice interior setting overlooking a golf course. So British, and at Malaysia prices (about $25 per person)

Public transit is quite poor in Penang, but if you are a foodie, Penang is like the foodie capital in one of the most foodie countries in the world

Kuala Lumpur has every Asian cuisine you can think of. If you want top quality sushi, you can find it. Sichuan cuisine is common. High quality Indian food is everywhere. Vietnamese, Thai, and more is easily found even in shopping malls. High quality western cuisine is lacking a bit (supposedly Bangkok is better in this respect), but KL has you covered on any Asian foods.
Because there are too many empty apartments in Kuala Lumpur (the Chinese overbuilt in Malaysia like they have done back home), the Airbnb prices are great, and even better, you can likely negotiate the price on longer stays due to the excessive supply of units
If you like parties, hanging out with backpackers, cheap alcohol, and social life, I recommend Thailand instead of Malaysia. If you like an organized and clean large city with good shopping and great value amenities (especially if you want to hunker down in your apartment to focus on work), I suggest Kuala Lumpur.
Kuala Lumpur may be the best value city in the world. I consider it cheap Singapore. The modern apartments in downtown KL have a pool and gym in the building by default. The public transit is great as long as it goes to where you need it to go. And the prices are much more affordable than Europe for almost everything.