I just read about your Kuantan option. The Hyatt beach is decent, not the best for really swimming but more like dipping your toes and get some salt water on your skin. The resort ground however is nice with a great pool. There are some local stuff to do if you have a car, e.g. Sungai Lembing former tin mine (they have a small cute museum and local restaurant around there with some hiking too).

If you want a better beach but it a more remote area, maybe you can drive a bit further north to Tanjong Jara Beach resort in Kuala Dungun. Whilst I haven't stayed at this resort before, the beaches in Terengganu are generally better than those in Kuantan.