For Grab you will need local SIM which you can get after arrival.
At Langkawi better to get your own rental car, last time when I was there (2014) it was like US $20/day.
There is a night market with great street food as well as well known small eateries.
Airport has just one terminal, customs/etc is quite fast. If you arrive 45 min before your flight it should be enough.
Don't know what you are planning to do in Kuala Lumpur, but I would limit my stay there for 24h max.