This study examined the impact of collaborative pre-task strategic planning followed by rehearsal on the quantity and novelty of content used in task performances when strategic planning is performed in different language conditions in an online classroom. Forty Japanese university students of English as a foreign language (EFL) from two intact online classrooms were separated into pairs and performed second language (L2) problem-solution monologue tasks in two conditions. The first condition involved collaborative strategic planning in the second language (L2SP), which was followed by a task rehearsal; the second condition involved collaborative strategic planning in the first language (L1SP), which was followed by a task rehearsal. Task performances were analysed in terms of idea units generated and, by examining strategic planning transcripts and planning worksheets, were coded for how they were prepared. As a measure of problem-solving creativity, ideas used in task performances were coded in terms of their novelty. Findings revealed a non-significant difference between the number of ideas used in each planning condition. However, performances that followed L1SP and rehearsal contained ideas that were significantly more novel/rare than performances that followed L2SP and rehearsal. The L1SP condition led to more planned and rehearsed ideas used in performances than the L2SP condition, which likely meant learners spent more time refining and formulating language to express L1-generated ideas, leading to more creative solutions to the task problem. A content analysis of post-task questionnaire responses revealed that learners perceived L1SP to be beneficial in terms of conceptualizing ideas while L2SP was perceived to be beneficial in terms of formulating language. Additionally, learners felt that online synchronous communication facilitated collaboration during planning. These findings are discussed in terms of using the L1 as a tool to enhance creativity in L2 tasks and using online tools to enhance collaboration during pre-task planning.