What to include in a letter of support for an academic assessment? I have been asked to write a short note that includes comments about the impact and importance of a colleagues' work over the course of the past year - for a pre-tenure review.

I could discuss a notable recent publication, a conference he is organizing, inspiring discussions we have had over the past year, and his public outreach at a recent conference. But it is not clear how I do so in a way that most effectively supports his work, and would appreciate advice from more experienced academics who request and review such letters. 

How can I most effectively frame and focus these points while keeping the letter brief and genuine? In addition to these points, what are the key features of such a letter?
I would suggest by starting with a general statement about your colleague as an academic (quality of work, productivity etc. relative to others, in general, in the specific sub-discipline) following that you could exemplify this with the points you mention. Tiyr second paragraph could almost be copied into such a letter but with some more detail. A second item may be to set the last year performance in perspective of the longer term. This seems not to be asked for but I think it is a valuable step to set the year's performance in perspective both relative to what has been achieved earlier and relative to what can be expected visavi the person's job siuation (teaching load, admin load etc.. For tenure, I suppose personal traits and activities within the department are not of interest since they would be known internally. Other wise such points should be included.