i have had very good luck at the following at that time of year: khao lak, krabi, hua hin, jontiem, and koh chang

The Southern Gulf Islands tend to be drier at this time of year. I have been to Samui twice in June and have had glorious weather.

I have also been to Hua Hin, which is a nice coastal resort, but less of a tropical feel than the Islands, in May for 2 weeks and saw only 1 day of rain.

Beaches and Islands on the Andaman Sea side, Khoa Lak, Phuket, Phi-Phi, Ao Nang(Krabi) are just entering their monsoon season, so you could be lucky, but if the rains come early, it could get very wet.

Here is a helpful page that gives you historical data for all of Thailand. You will see that everywhere gets considerable amounts of rain in June, but dont compare it to rainfall figures at home. Rain in the tropics tends to fall in shorter, intense, periods and then clears and drys rapidly.

http://www.travelfish.org/weather_fish.php

Splurge for Bangkok = The Mandarin Oriental or the Peninsula hotel. Both on the river and romantic. Non riverside hotels that are considered very good = Four Seasons, Siam Kempinski, Hansar and Grand Hyatt.

For beaches, IMO it's all a gamble with the rain but for late May early June you can have good weather. August and September are the months with the most rain IMO. The Krabi - Khao Lak area is nice and lots for you to do around there. Koh Samui is small and I would get bored with 5 - 6 days on the island. Do note that there are only a few beaches with fine soft sand on Samui and the rest have either rocks or muddy bottoms. You could do a package with the Four Seasons combining Bangkok and Samui.

Hua Hin is charming with nice beaches but gets crowded on weekends and holidays. The best hotels in Hua Hin are Chivasom(health retreat), Grand Hyatt(Barai), Intercontenental and there are a lot of smaller resorts as well.

In general don't worry too much about the weather, unless there's a major storm or fronts, and if you get rain it will only be for a few hours a day.

Okay, thank you so much for all of your advice! I ran into someone today who's been to Thailand and they were trying to talk me out of going in May/June. They said it would be too steamy hot to enjoy it and rethink the trip for November. We haven't booked the tickets yet but I sure would like to go sooner rather than later! Plus, the entire point is celebrating my birthday (which is in June).