Advice for Vietnam, Cambodia and Thailand?

Two of my good friends are planning a trip to Vietnam, Cambodia, and Thailand this coming March. Does anyone out there have some good advice for them? Specifically, they are looking for the following information:

1. Siem Reap - Does anyone have a great English-speaking guide for Angkor Wat, Angkor Thom, etc.? We will be in Siem Reap for 3 days.

2. Ha Long Bay - We booked an overnight boat trip through Inserimex Travel, who we were referred to by John Gray's Sea Canoe. As far as I can understand, the boat is not the most luxurious and we will be the only passengers on it, but it is the only option that takes people kayaking deep into the caves. I would prefer a bigger, air conditioned boat with other tourists on it, but don't want to sacrifice the opportunity to kayak into the caves. Thoughts? Recommendations?

3. Chiang Mai - I've heard that the Four Seasons arranges wonderful hikes in the area. We are not staying at the Four Seasons. : ) I've also heard that visits to towns around Chiang Mai seem staged (because they are). Any recommendations for good guided tours or hikes in the area?