Grenada - the beaches are just stunning, stay by Grand Anse and visit nearby Morne Rouge beach. You can do snorkelling trips including the unwater sculpture park, Chocolate plantation/factory, Georgetown, Waterfalls, Grand Etang NP etc. Definitely better than St Lucia, fly with BA.
Mexico is amazing but the sargassum on the beaches can be a huge problem depending on when you go. Personally I wouldn't stay in Cancun or PDC, Akumal is much nicer but depends what you like/can afford. Chichen Itza, Coba, ceynotes etc so much to see and do. Good buses too.
Bali has fabulous temples - but are the teens going to care? IMO Kuta is a dump - Benidorm for Aussies - but depends what you like, might appeal to teens. I stayed in Ubud which I liked a lot, but inland. I wouldn't go to Bali for beautiful beaches, you need to go elsewhere for that, the Gilis i think are popular but not sure how over touristed they are now.
14 days on an island in the Maldives wouldn't work for me, way too long on a small island, I'd be bored sick even with snorkelling and a nice island. I don't think island hopping is easy either because you often have to go back to Male before you can go to the next island.
The same goes for Dubai - I couldn't spend 14 days there. I'd have to combine with Oman and probably Abu Dhabi as well for that amount of time.
Thailand is fabulous and the Thai people wonderful. Getting around isn't as easy as some other places though IMO and it's a long country so distances can be very large. By train Chaing Mai to Surat Thani (for iKrabi etc) require two over night trains for example. Sex tourism obvious in certain places. Bangkok is a fantastic city but can be stressful with tuktuk scams etc. The river is fab though - take the public boats for very little money and there is an MRT and sky train system. It's a spread out city though, not one for just strolling IMO. Then so many beaches and islands to choose from....