I can remember going to many of these when I was a young kid. Each year it was fairly similar, though I think there were a few things they didn't discuss until the later years (grade 5-7). I can remember doing the same activities as other posters have described, so I guess there really hasn't been much change over the years in terms of how they are trying to convey their message to kids.
I can specifically remember one activity that we had to do, where basically a few students were randomly selected (myself being one of them) and the person running the session pretending to be a "peer" trying to get us to smoke a ciggarette. It was our task to practice how to reject this kind of offer. Being the smartass 5th grader that I was, my reply to this was something to the effect of "I wouldn't waste my time with tobacco because pot is much better". The look on the teachers face was priceless (if anything I guess she was shocked to hear it from such a young child). That said, the attitude towards pot during the 90's in SA was far more relaxed than it is now (and the laws reflected that given that we could grow 12 hydro plants and only get a $150 civil fine).
I can't really remember any information being given that I'd associate with harm minimisation - it was all pretty much of the "just say no" style. That said, I can't imagine 3rd graders would need to be educated about things like how to use/dispose of a fit correctly, or how to test pills for adultrants. Admittedly though there's probably some more relevant HM techniques that could be taught to children that I've overlooked.
edit: Actually now that I think about it, there was one activity that could have been considered HM. It was designed to show us the legal limits for drinking and driving. Basically we were shown a chart that had what quantities of particular alcoholic beverages (wine, beer, vodka etc) made up one standard drink. We were also told how much one could drink before being over the legal limit, which at the time was "4 standard drinks for male and 2 for females". That was a little confusing however, given that there was no time limit indicated (4 standard drinks in 10 minutes or over the space of 4 hours?). I'd imagine its different these days - to my understanding the law now is (for SA atleast) that you can have 2 standard drinks in the first hour and one every hour after (or one in the first hour and one every hour after for females).