We investigate how to design community technologies for public events. We do so with a focus on technologies that give rise to new forms of participation and knowledge coproduction in public libraries. Specifically, we deployed a digital service at a major public library during its four-week creative workshop series. The system offered an alternative way for people to work together as a community, to go beyond achieving individual goals, and to contribute to the achievement of public goals (e.g., building community bookshelves). We report on how the system has reconfigured physical spaces and afforded new social practices in the library. We propose Computational Alternatives as a fruitful approach for gaining situated, nuanced insights into a technology's possible adoption. We offer key insights in the form of computational alternatives vignettes-grounded stories that encapsulate sociotechnical implications of technology, pointing to plausible alternative futures.