Background: Previous studies have investigated effects of out-of-school STEM activities aimed at stimulating children's interest in science with positive results. However, research has not discussed the reasons why such activities are successful. Purpose: In this study, we address this gap by investigating which factors children themselves identified as interesting when they visited events at an out-of-school activity named The Children's University. Sample: Children aged 8-12 participated in the study. Altogether, there were 353 children involved in the data collection. Design and methods: A mixed method design was used, including a questionnaire and semi-structured interviews in which children's self-reported experiences were collected. Likert scale questions in the questionnaire were analysed based on descriptive statistics. The open-ended questions and data from the interviews were categorized by content analysis and analytically interpreted through 'the Ecological framework for understanding learning across places and pursuits'. Results: The children were positive about their visit, and these utterances could mainly be related to the development of the individuals' interest and knowledge according to the Ecological framework. We identified two new factors influencing student's interest in STEM in out-of-school activities: appreciating the spectacular and learning; verifying two factors of importance previously suggested in the literature: appreciating the content and the learning environment. Conclusions: The study highlights the specific factors the children actually appreciated from their visits to out-of-school activities, which could be of interest for stakeholders arranging different kinds of STEM events promoting informal learning. The content in the activities is important as well as spectacular features. To have the opportunity to learn something new in an environment that is conducive to learning is also of importance for children.