Not for beaches but have you considered Malaysia (other than Langkawi which you've been to)?
We like Sabah for the wildlife and culture, particularly if you don't mind a Four Points (Sandakan). LM Kota Kinabalu is one of our favorites. There are also the two Marriott resorts which I know nothing about but look OK.
Also Penang is wonderful for culture/exploring. Macalister Mansion in Georgetown is well situated and nicely upmarket. For a beach you'd have to go to Batu Ferringhi and Hilton. We like the Four Points Penang occasionally but it can get busy and isn't great location-wise.