I live in Kuala Lumpur all my life and here are my recommendations:
1. For Drinks, definitely Changkat, Bukit Bintang. Jalan Nagasari is here too. Depending on what you like, the mainstream Changkat, check out Havanna. For Jalan Nagasari, there's a few small hipster bars/coffee shop. No Black Tie if you like jazz.
2. Tourist stuff, it's fun:
Check out the flying rotating tower called KL Tower.
Batu Caves (this is actually a cave on top of a mountain) and pet the monkeys, eat at the vegan restaurants at the bottom of the hill and order a "Roti Tissue". (yes that's how they pronounce it)
This is Roti Tissue
3. China Town!
Go there and buy cheap shit you'll throw away a week later. Food is great but I personally don't eat there.
4. After a night at Havanna clubbing and hitting on chicks, don't forget to eat at the stall that sells nice seafood there. It's like a van converted into a BBQ boiler thing. Lok Lok Steamboard (it's a cantonese word)
5. The Rainforest is supposed to be nice. I've never been.
6. Perhentian Island. YES. Probably one of our best islands. I wouldn't go during the rainy season though. You'd just stay indoor.
7. Try the Nasi Lemak. The best one is at Village Park Restaurant at Damansara Uptown, this is hard to get to if you don't have a car and they're only open for lunch. You know they're the best when 2-3 of the prime ministers eat here.
8. Try the Mamak. I know for a fact that all foreigners(white people) love CHEESE NAAN!!! Try it, this is something you can't get anywhere else.
9. Bangsar Village 2. Check out the outdoor area, good food, good bars. Bangsar is like the uptown/posh area. Make sure you look at things on the 2nd floor, they're hidden, but good. Get food from Wondermama and request to sit upstairs, it's nice.
I have more recommendations as I have a lot of expat friends maybe I'll post them later.
One request I have is to always be courteous and polite to people when you here. I'm sure you'll have a good time.
p/s: You must download an app called MYTEKSI! This is the best AND safest way to get taxi in Malaysia.