You could incorporate the anniversary into other guild events throughout the year. Maybe if you do a challenge quilt, you can use Silver or the number 25 (25 blocks or 25 inches or 25 something else...) as the the theme of the competition. Maybe each monthly meeting could have a founding member/committee chair highlighted or their own trunk show or something. Are there any games or activities that have already become tradition for your guild, that you could modify to highlight "25"?