Early May isn't the best time for Krabi/Langkawi (or the west coast). It's the start of the rainy season and seas can get a bit rough then. Samui on the east is probably better if you want a nonstop flight from SIN.

Alternatively, you can visit most Indonesian islands south of the equator (Bali, Lombok, Komodo etc.) or Malaysia. Perhentian or Redang requires a flight to Kota Baru or Kuala Terengganu and then a Grab (uber equivalent of southeast asia) ride to the jetty (not a nonstop flight - it goes via KUL or you can take a nonstop from Johor Baru across the border). There are nonstop flights from SIN to Kota Kinabalu and you can go to Gaya Island from there (15 minutes Grab ride to jetty) - surprisingly decent for an island considering how close it is to a huge city. Best islands in Malaysia would be those off the Semporna Coast (Mabul, Mataking, Pom-pom etc.) but requires a transfer at Kota Kinabalu or KL.

Philippines is an option too. Take a flight to Cebu and then grab + ferry to Bohol.