There's plenty to do in the airport for 3+ hours, but if you are curious, you can take a taxi ride over to the Sepang circuit racetrack. It's within throwing distance of the airport. You can see it from outside the terminal, but take a taxi. This is where the Malaysia F1 Grand Prix is held. There is generally other activity going on at the race track. You can check online to see what's happening at the time you will be there, or you can just tour it in ghost town mode if there is no activity. I saw the F1 race there in 2000. Great seats at the start/finish line with a straight view of the Ferrari and McLaren pits. Michael Schumacher of course won for team Ferrari.

If you take the 5+ hour option, then you can get to KLCC by taxi and back. You can see the Petronas towers from the outside, but I think the touring policy has changed since I was last there. You can see the towers, walk the loop of the KLCC park and grab a bite at the KLCC shopping center.

M8