what to do and see depends on whatever floats your boat. we should be in Samui up until the 1st June have just booked flights back to bangkok on that date.
Have you looked at villa rental with there being 6 of you?
Don't know if that will work out cheaper or be better or not. May they have some good deals. The Amari offer a free room for kids accompanying adults in May so would be worth checking out. lovely hotel the Amari We can recommend it.
Do you want to be in the thick of it?
some great eating places on samui as well.
you will love your holiday I am sure.

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Kiss
God laughs at people who make plans.