don't say 'just a quick note' ... that's exactly what you DON'T want it to seem like - it could make it sound like you just scribbled it between the ad breaks on the back of an envelope or something.
say, 'i just wanted to send you a letter to say thank you for ...'
also say it is so nice of him to think of me,
he was a great uncle,
you will miss him (if he was and if you will, of course)
maybe mention what you might do with it ... (probably best its nothing too frivolous though, like spoiling the kids or yourself, or handbags or shoes etc - maybe say you will put it towards some lessons or a much needed new washing machine, or new car or whatever - make something up if you don't know)
also why not 'flesh' it out by including a few copies of some photos of him, ones she doesn't have, say you thought she may like them.
also, depending on your location and relationship with her, why not finish off with an invite for lunch somewhere?