Much as I love KL, I wouldn't recommend a transit visit unless you have something very specific in mind. It's not a wanderer's city: and unless you collect sights like the Petronas Towers or enjoy upscale shopping malls, there's not much for the casual visitor to see.
On the other hand there are spots like the Islamic Arts Museum (
www.iamm.org.my) which has some stunning exhibits (and a good restaurant). Or the Batu caves. Or even the Suria Mall (Petronas towers) if that's your thing. Tripadvisor might help you decide.
But wherever you go, consider a taxi from the airport directly to your destination. Batu, or a central KL spot, should be around an hour/hour and a half. For two of you it will be cheaper than train, and you'll avoid the faff and problems associated with taxi transfers from the central station to wherever you are going. The penalty is traffic at peak times.
An alternative would be to get a day room and relax, use the pool. Prepare for your onward flight ....
Oh, and the lounges are absolutely fine - but can get crowded. The F section has a restaurant, but that too can fill up.