Frequent engagement in English extramural activities (i.e., activities that take place outside the classroom) has been found to have a positive impact on EFL learners’ vocabulary knowledge and reading comprehension. In the present study, we aim to extend our knowledge of the possible impact of extramural activities into the realm of second-language writing. Specifically, we investigate the relationship among a number of English extramural activities and two aspects of writing development: lexical diversity and noun phrase complexity. The data are drawn from the Swedish Learner English Corpus (SLEC) which includes texts produced by Swedish secondary school students. The corpus also includes information on how many hours per week students (i) engage in conversations in English, (ii) communicate in English while playing computer/video games, (iii) read in English, (iv) spend time on social media with English content, (v) and watch TV shows or movies in English. The results show that reading in English leads to higher frequency of adjectival modification, whereas conversing in English and watching TV programs positively impact lexical diversity. The results of the study have implications for discussions about the role of L2 classroom instruction vis-à-vis learners’ extramural activities.