" ... A thank you can be simple and it doesn't have to spoken--a wai, putting it on, an expression of how good it will look, or something ... "

Actually I'm a bit surprised at this response. I know what Scott means about "thank yous" not coming at you verbally, but I would expect that a small wai would be a pretty common, almost kneejerk response to a gift. I get one every time when handing over something designated 'gift' (as opposed ~ as Scott says ~ to a dinner, or a night out, or a taxi ride ... everyday occurences those). Not speaking from no experience here . . . after 8 years together with the same Thai guy I've certainly had enough experience with 'gifts'

As far as the original question is concerned, one gift always appreciated is a few thousand baht specifically for him to give his parents. If you state what it's for most Thai guys will take great pains to see that it gets there.

If wais count as acknowledgment and gratitude, that probably covers it. My students wai if you give them one chocolate candy.

As a visitor to Thailand, not a resident, I have always assumed that one of the uses of the wai was to say 'thank you'. I certainly find a broad smile and a wai an adequate replacement for the verbalisation that is customary with westerners. The politeness of not opening a present in the presence of the giver avoids the sometimes clumsy effusiveness we are often forced into or feel is appropriate in the West; and it's obvious Thais don't feel they have to mention the gift the next time you meet them. However, I have found that the thanks is often expressed in action. A motorcycle helmet I paid for as a gift to a friend's bf was worn with pride and a smile when I arrived in Thailand; one of the shirts I brought him from the UK was worn the first night we went out to dinner together; a cash farewell gift, handed to him in an envelope, was acknowleged, as I left the condo building, with an appearance on the balcony, a ravishing smile, and his totally personal way of waving goodbye. (Just to prove how much the gift was appreciated, my friend told me his bf waited a tense but seemly half-hour before rushing to Tukcom to turn it into the mobile phone that everyone had to have that week.) Personally, I consider myself fully thanked with a smile, a wai, and behaviour that expresses, without words, that the gift has been received gratefully and is being enjoyed.