My cousin was going to go to Thailand with her boyfriend, but he ended up not being able to go, so now I'm going with her, yay for me!
She already has the whole trip planned out pretty much, but I'm sure I she'll be accepting to ideas and input from me (or should I say, from YOU).
We mostly like the outdoors, hiking, backpacking, snorkeling/diving, kayaking, etc. I'd like to stay away from the REALLY touristy stuff, but I don't want to miss out on an amazing place just because lots of people go there. So here's the trip so far, tell me what you think. Oh, and the North isn't an option since we aren't gonna be in Thailand that long.

Arrive in Bangkok, stay in Bangkok 4 days.
Take an overnight train to Phuket where we'd stay 2 days.
Stay in the Koh Samui and surrounding island area for 4 days.
Take the overnight train back to Bangkok where we'd spend the night.
Then head to the border of Cambodia where we'd cross and spend the night in Siem Reap. The next day we'd tour Angkor Wat, and spend another night there.
Then it's back to Bangkok, and to the airport the next day.
Then home.

Questions:
Too much time in Bangkok?
What to do when in the tourist trap that is Phuket?
What islands in the Koh Samui do you recommend? And any activities? Any places? Too little time in the island area?
One whole day plenty time to see Angkor Wat?

Any other recommendations??

Thank you very much, in advance!

To answer your questions in order:
(1) No, four days in Bangkok is not too much. It is a big sprawling city with much to see and do. I love the place, others are repelled by it. By the time you do the Grand Palace, the Chao Phraya River, Wat Arun, various stupa and Buddha monuments, visited Sukhumvit, Khao San Road as well as other places, you will only have scratched the surface of this fascinating city.

(2) Two days in Phuket would two too many for me, however, you like snorkeling and diving and the Phang Nga bay area is made for that as well as several spots on the Andaman.

(3)As for the tourist trap of Phuket, my advice would be to give a wide berth to the gutter town of Patong. If you do that, your two days in Phuket will be immeasurably improved.

(4) Four days in Koh Samui and just the island itself would be on the short side, however, your time is limited. There is fantastic diving/snorkeling in the National Marine Park just off the coast of Koh Samui. It is truly beautiful. There are many tour companies on Koh Samui who will arrange a snorkeling/diving trip for you with a visit to a couple of outlying islands thrown in.

Chaweng to my way of thinking is akin to Patong in Phuket, but not as bad. Bophut in the north of the island and Lamai to the south are far better places.

Ankor Wat and only one day? Ankor Wat is not only about the stupendous temples, but it is also a place in Cambodia. Two days should be the absolute minimum and three days ideal. However, once again, I recognise that you are strapped for additional time. Better to see part of Ankor Wat than not at all.

Finally, you could increase your leisure/tourism time by cutting out the trains. Train journeys in Thailand are notoriously unreliable owing to the condition of the tracks. The trains seldom arrive on time and that will eat into your holiday as well. There are a number of budget airlines operating in Thailand and their cheap seats make them an ideal option for journeying around the Kingdom, rather than by some unreliable trains.

Just my suggestions, others may disagree. Have a safe journey.

Looks like a sound itinerary. Horses for courses really, but even though I do like Bangkok a lot, I would trim a day off your first 4 days there and distribute that to one of the islands instead, since you like the outdoors/snorkelling etc and will be passing through Bangkok twice more.

I can recommend cycling around Samui. Outside of the main centres the roads are fairly uncongested and it's a nice cycle. Stop off at beaches or restaurants along the way.

A whole day - sunrise to sundown - was a good sampler of Angkor Wat for me. Even though it is a sprawling place I got to see a good cross-section of temples, including the main (read: popular) candidates. To be honest I was more than a bit templed out after all that, and the tourists got on my nerves after a while (as did the beggars/sellers between temples). So I am not sure I could've done three days - especially at $20 a day.

Have a good trip and pop back when you have more questions or even recommendations for us all after your trip...

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Thanks for the response!
I've talked to my cousin since I've posted this, and things have changed a bit. She'll have already been there 2 weeks before I arrive, so it looks like she'll have gone to Phuket already, which means she wouldn't want to again, and I really wasn't attracted to Phuket, so that's good news, we probably won't go there at all.
Also, the last time I talked to her it sounded like we'd be taking planes all over the place instead of the trains.

I spoke with my cousin again.
When I get there, she will have been in Thailand for 2 weeks already, she will have gone to Phuket in that time, and I don't know where else, so I think we'll skip Phuket when I'm there. It also sounded like we'd be cutting one day out of Bangkok on the first stay. Another change is that we will be going to Angkor Wat first, then seeing other parts of Thailand. She's told me that I can pick other places to fill our itinerary, so now the North is an option.
I've heard the Khao Sok National park is really nice, and it's by the Koh Samui/Phangan island area, have you guys been there?

Would you recommend visiting the North or visiting more places in the South, or something else?

Thanks!

Oh, and we will mostly be traveling by plane instead of train.

The north has a lot to offer with Chiang Mai, Chiang Rai and Pai as the main settlements/backpacker spots. The highlands are stunning (Doi Inthanon for example) hiking country.

I was going to say that you can't get a direct train to Phuket. You'll need to bus over from the east coast. If you go south there are some lovely islands further south on the west coast. Koh Lanta and Koh Lipe for example. Less touristy than the islands further north but just as beautiful, if not more so!

I know you want to fly but I would recommend at least one train journey for the experience. you can take a train south from Bangkok through Chumphon (where you can ferry over to Koh Tao), through Surat Thani (ferry over to Samui/Pha Ngan) and then on down to Hat Yai and on into Malaysia. Bangkok to Surat Thani is around 13 hours and you can do it overnight.