Say 9am flight, arrival 10am, pass immigration at 11am. Take train from KLIA2 to KL Sentral (40mins). From KL Sentral one can Grab to Midvalley Mega Mall, shop & makan. Back to KL Sentral and take LRT to KLCC, shop, sight see & makan. Back to KL Sentral, shop & makan at NU Sentral and travel by train back to KLIA2 and more makan before return trip
But seriously, if tired of shopping & makan, can take train from KL Sentral to Batu Caves and back, that is probably like a day trip already. I normally used KL Sentral as the main transit point.