For a half a day, I would highly recommend the following:
(1) Definitely visit the Petronas Twin Towers. They are architecturally stunning. At the base of the Towers is a shopping mall called Suria KLCC. There's a great food court and some good restaurants in the mall. Next door is the Mandarin Oriental where there is also good food to be had. You can spend an entire half-day here for sure.
(2a) For the best view in the city, go up the KL Tower / Menara KL. The panoramic view is far better than the limited view you get from ascending to the Petronas Twin Towers' Skybridge.
(2b) For souvenir shopping, go to Central Market. Lots of Malaysian handicrafts for moderate prices.
Enjoy your short layover!
