Great idea John, in my experience donors love an acknowledgment of this kind however prefer not for the gift to cost too much. I’ve also done similar acknowledgements for reaching Lifetime Value milestones. Depending on your organisation and the overall levels of giving have hosted a small lunch for a $1Million milestone (this was a previous organisation I worked at).
According to the level of support I usually begin with a certificate or handwritten card. If a scholarship donor, perhaps a quote or artwork (if relevant) from their student printed or framed. Small gift, for example, here at St Hilda’s, we have really beautiful looking (and scented) candles that would be a lovely gift for such an acknowledgement.
Hope that helps. What are you thinking of? I also have a stewardship matrix I’d be happy to send to you if that is of interest. Which is based on levels more than years of giving.