I recently shared my experiences for my July trip to Kuala Lumpur here. Overall, I thought it was a diverse city with great food and a great nightlife. Though since it was my first time in Southeast Asia, I found it a little overwhelming at first. I think a total of 2-3 days are enough to explore all of KL before you venture off to other Malaysian cities. Here are a few tips that might make your planning easier:
1. Climb the 272 steps up to the Batu Caves under the shadow of an enormous golden Hindu statues while you interact with and avoid bites from the playful monkeys.
2. KL's public transportation system is relatively developed, but not always connected. It's a cheap alternative to taking taxis, but taxis aren't too expensive either.
Note that most taxis will not use the meter, and will set a fixed fare before the trip. They will most likely try to rip you off, so be firm when you negotiate and refuse to ride with the drivers that won't go lower. You can ride the next cab that comes around. (RM 10-15 should cover most cross town trips around 15 minutes, even with traffic.) Most KL natives speak English, so you won't need to worry about getting lost in translation.
3. Explore the streets of Chinatown and eat awesome (and cheap) street food. Buzzfeed's article highlights the must-eats.
4. If you don't fancy street food, here is a list of Malaysian foods so you know what to order at the local restaurants. Hint: a lot of them are different variations of curry or fried chicken.
5. Make reservations at Marini's on 57 and enjoy the rooftop bar's amazing views of KL, especially the glowing Petronas towers, at night. If money is not an issue, make reservations at the restaurant for great Italian food instead of at the bar/lounge. Another great rooftop bar with an amazing view is Sky Bar, on the 33rd floor of the Trader's Hotel.
6. The Petronas Twin Towers are always worth a visit. although, if you want to save money and still enjoy the view of the city from the towers, I suggest going to rooftop bars instead. There is also a shopping mall area at the base of the Petronas Towers.
7. Visit the KL tower--but the same advice as with the Twin Towers. You don't need to spend money to go all the way up in order to enjoy a great view of the city.
There are also a lot of foreign restaurants and bars in this area, such as Hard Rock Cafe and TGIFriday's, if you're looking for American, Italian, Japanese, etc. food.
11. The Thean Hou Temple holds beautiful Buddhist architecture and views of the city. Entrance is free! It's open until 9pm and at night, the temple glitters from the light of many pretty Chinese lanterns.
12. Go people watching at the Bintang Walk. It gets livelier at night with street shows and performers. As always, be careful of scammers and pick pocketers.
Bintang Walk is also near all the large shopping centers, such as Bukit Bintang Plaza, Berjaya Times Square, Kuala Lumpur Plaza, Imbi Plaza, etc.
13. I suggest around 2-3 days to explore all of KL. There are also great day trips to get away from the city.