Been there - done that! In almost every city there is a Chinatown to explore. KL's is not amongst the best, specially if compared to Singapore and other cities, but it does have a few special spots to explore.
Petaling Street for me is a tourist place with lots of stalls to buy stuff that you can also get somewhere else, but with the combination of eateries, temples and clan houses it still has a good flair. I usually combine a visit to this area with visiting the historic district of KL, or at the early morning or late evening of another trip by monorail, which stops just around the corner of the Guan Yin temple.
During my last visit to the Guan Yin temple I asked for "joss sticks" for my prayers and the otherwise inattentive ladies at the desk asked back "chop sticks"? So, next time I better ask for "incense sticks", I guess :-)
Besides an interesting and colourful architecture, which needs some renovation badly (again compared with rich Singapore) there are several Chinese buildings of interest, like the Guan Yin temple, the Chinese Assembly Hall, the awesome Chan She Shu Yuen Association Hall and one of my favourite temples, the Sin Sze Si Ya temple (which I will cover separately). The Sri Maha Mariamman Indian Temple is great to explore too, but do not forget to take off your shoes and leave them OUTSIDE at the window. I wasn't sure if photography was allowed here and since I didn't want to piss off someone, I didn't take any photos at all.
Most tourists will come here though, for food, drinks and cheap deals for shopping.