The Edublogs site has an overwhelming number of ideas for an educational blog. Subject areas, web 2.0 tools, and suggestions for use are well set out and easy to navigate. For a school library in particular, commentary around books including opinions, reviews, redesigning covers, adding extra characters and comparisons, would be a natural place to begin.
There are two ideas listed on page 5 of the pdf version that I particularly like. Activity 7 is to write about a hero or mentor. Activity 8 is to write about your country or another culture you are interested in. Both of these posts would encourage parent involvement and give students quite a lot of freedom in how to answer the question. The online prompts on page 7 have a multitude of ideas to simply write.
The edublogs site is a valuable guide and bibliography of ideas for activities for a virtual space in a classroom or library.
Morris, K. Burt, R. Waters, S. (n.d.) 150 student writing prompts for blogs, digital portfolios, or websites. The Edublogger. https://www.theedublogger.com/prompts-student-writing/#Web-Tools