Be careful of volunteer programs that want a lot of money. Many of them are run for profit only.

Try to choose a program where you will be providing something that isn't readily available from the Thai population. People who volunteer to do things like paint a school usually spend enough money getting there to hire local workers to paint every school in the area.

My recommendation would be teaching English, especiallly in some of the refugee camps on the Burmese border.

All of this is my personal opinion, of course.