I moved to Thailand at the end of last summer although I’m in a much quieter part and haven’t got out and explored too far yet. I’d been here a couple of times before and done a very nice holiday in Bangkok, Chiang Mai and Koh Samui which gives city, jungle and beach options and it’s easy to fly internally between them.
There are elephant sanctuaries all over but if animal welfare is a concern make sure to read reviews as some are far from being sanctuaries, same with the tiger ones and I personally tend to avoid those places. But genuine ones do exist.
Chiang Mai is great, there were loads of markets there when I visited and various jungle activities like river rafting, atv riding, elephants, golden temples etc. I hear good things about Chiang Rai too.
For beaches any of the main islands should give you that - from Samui you can get to Ko Pha Ngan (popular with backpackers for full moon parties), Ko Tao (good diving/snorkelling) and other smaller islands such as an amazing one I visited called Koh Nang Yuan near Ko Tao which had white sandy beaches, some of the clearest waters and easily the best snorkelling I’ve done. Krabi, Phuket, Phi Phi islands all offer similar, Hua Hin too. Plenty of other beach/island options too.
Bangkok is great, it has one or two sleazy parts but plenty of attractions and stuff to entertain. Traffic is frequently crazy so if you stay there then a hotel near the sky train can be useful.
Enjoy!