I love KL - are you open to any sort of ideas? Because I'm procrastinating, here's my ultimate guide to short stays in KL.
Lake district
I'd suggest a day at the lake district, just west of the city. I walk there and back from Masjid Jamek or Sentral (train stations) but taxis are always available (though sometimes it can be hard to get one to use their meter on the way back). My favourites are:
Bird park: RM42 each, I think there's a camera fee of RM1 as well. Open from 9am-6pm. Expensive, but great variety of birds. The hornbill restaurant there actually is not bad - meals aren't as overpriced as you'd expect with a captive audience. Don't miss the eagle feeding time! It's on at 2pm from memory, and there might be a morning one too.
Butterfly park: about RM17 each or so from memory with a camera fee. Better value for money than the bird park I think, but recommend both. It's not just butterflies there - got nice turtle ponds and have heaps of caged reptils, amphibians and instects.
Orchid + hibiscus garden: both nice ways to spend some time. They're free except sat/sun, then they're only a ringgit or two.
Just strolling around! is a nice thing to do in the district. Cooler and quieter than the city. Monkeys wandering around (particularly around the bird park netting) which is a novelty. I've never managed to get to the planatarium while it's open :-| but there's some nice views around the front, with a nice fountain display. There's a sculpture park, and the national mosque (masjid negara) is an impressive sight, and is great to hear the calls to prayer ringing out across the gardens. There's also museums, like the police museum (?!) here, but I've not been in any.