I'd recommend Typhus vaccine, and just stay away from stuff washed with normal water and selfmade icecubes... I had 2 food poisonings in Thailand because of that and that's really annoying ^^ I wish you a really nice trip! Thailand is awesome

Cambodia changed my life! Siem Reap and Angkor Wat are a must. If you're in need of some western comforts, Pub Street has some amazing western food options as well as local cuisine. It's a good area for nightlife too. Say Hi to Linda if you happen to cross paths with her! She's a local girl who sells bracelets to find her studies at an international school. She's famous around the area.
We spent time in Kampot/Kep which is close to the Vietnamese border. I LOVED Kampot. Kep was more desolate but if you love abandoned houses being taken by nature and crab, Kep is your place.
Phnom Penh is a great transit point but I wouldn't spend too much time there. It has similar vibes to Bangkok, just more expensive.
I spent a lot of time on the islands off of Sihanoukville. Koh Ta Kiev is one of the last remaining undeveloped islands, but there are three or four guest houses that have either camping or basic accommodations. If you want a 0mph island experience with the most stunning sunrise and sunsets minus the tourism, the Cambodian islands are phenomenal.