I usually get stopped in the shopping malls by people collecting for charities such as orphans, deaf, blind or elephant farms. I donate money, but politely decline the goods they offer in return.
There is a very old woman who I encounter occasionally, walking the road between my GF's village and her home village. Compared with her face, a prune is as smooth as a baby's bum. She goes between villages to help with funerals, for which she is paid in food. Every time I see her, I stop and give her 100 baht. She is delighted, every time. I can't understand a word she says.
I suppose helping people who are less fortunate than me might be making merit, but I don't know.