KL is two or three days. Not much to see or do. MO or RC, no other choice.
You could hop over to Kutching as do a trek in the jungle, see some wildlife and see a very different part of Malaysia, plenty to do for three days or more.
Hop in a car and go to Malacca, for the day. Penang is also possible but little to see. Flights in Malaysia are not expensive.
You could just stay longer in Langkawi. If not the FS than the Datai is nice but do not stay in the main building. Tanjung Rhu is not the best service (but far from bad, a little slow), setting is nice and they limit the number of kids. They have good rates on their suites.