Based on your search for free STEAM-based activities, I recommend using Quizizz. This platform offers a wide range of features that can help you create engaging and interactive STEAM activities for your students.

With Quizizz, you can create gamified quizzes and lessons that can be paced according to your or your students' preferences. This can be particularly useful for STEAM activities, as it allows you to incorporate elements of play and competition, which can enhance student engagement and motivation.

One of the standout features of Quizizz is its AI-Enabled Quiz Generator. This feature can automatically generate quizzes from existing documents or text, saving you time and effort in creating STEAM activities. You can use this feature to quickly create quizzes based on your STEAM lesson materials.

In addition, Quizizz provides access to millions of pre-made quizzes that you can customize to suit your specific teaching needs. This means you can easily find and adapt existing STEAM quizzes to fit your curriculum.

Finally, Quizizz generates detailed reports on student performance, which can help you monitor your students' progress in STEAM subjects and identify areas where they may need additional support.

While Miro also offers a range of features that could be useful for STEAM activities, such as its collaborative whiteboard and templates for creating mind maps and flowcharts, it may not be as directly suited to your needs as Quizizz. However, it could be a useful supplementary tool for facilitating group projects and brainstorming sessions.