Obviously temples are a big part of Thai culture and so will likely be a big part of your trip. You won't be able to, or want to even, go to every single one but you should definitely make time to see the more unique ones. Some of these are listed below along with some natural wonders.
The Grand Palace, Bangkok
The home to the Thai royal family is a big complex of buildings in the centre of Bangkok and one of the main tourist attractions in the city. You'll need to be modestly dressed, if your clothes don't meet the requirements, don't worry they will have some very fetching clothes to lend you. You can spend ages walking around the whole complex, make sure you also visit the on site temple Wat Phra Kaew, home to the Emerald Buddha and the Queen Sirikit Museum of Textiles.
Wat Pho, Bangkok
Wat Pho is another impressive temple in the middle of Bangkok, particularly for the large, golden reclining Buddha at its core. Again there are lots of little features and details you will find whilst wondering around. Remember to take your shoes off before going inside the temples and don't point your stinky feet at anyone - it's rude.
Wat Arun, Bangkok
Another temple but this ones a very different style. Go in past the golden Buddha and climb the stone steps to the top of the temple to be greeted by views of the city and the river. The steps are very steep and there is not much to support yourself on other than a bit of rope so it can be a bit of a challenge especially in the hot sun. Best way to get here is just by getting a river taxi for 3 Baht and there is a nice market nearby by selling food and stuff.
The White Temple, Chiang Rai
The White Temple which is near Chiang Rai in the north of Thailand is actually quite new. Real name Wat Rong Khun, it opened in 1997 after an artist rebuilt the original temple that stood there. It is very impressive and is continuing to expand. Look closely at the walls inside the main building and you will see some of your favourite characters including Superman and Neyo from The Matrix.
Maya Bay, Koh Phi Phi
Quite possibly the ultimate beach, Maya Bay is so beautiful that it played the part of The Beach in the film The Beach and a very good job it did too. You can go on day trips here via long tail boat which also include snorkelling as well as a nice ricey lunch.
Doi Suthep, Chiang Mai
Doi Suthep is way up on a hill over looking Chaing Mai. It has an impressive staircase up to the top where you'll be welcomed by its many golden structures and a series of great big bells. Best way to get here is to get the tuk tuks that look like mini red buses as they are the cheapest and just pile everyone in. Catch them from the centre of Chiang Mai.
Cheow Lan Lake, Khao Sok National Park
Cheow Lan Lake is a man made reservoir part of Khao Sok National Park. The cliffs arising from the still water are a sight to behold and best explored by long tail boat or Kayak. There are a few different floating raft houses set up along the lake so you can wake up to the misty mountains in the morning and go for a swim. These places often provide Kayaks for you to go out exploring by yourself or guided tours via long tail boat to see what wildlife is around.