I have used some of the activities mentioned by posters in this forum in the past- such as floating and sinking and making a volcano.
We too would try to implement STEM across the curriculum and have found that the pupils engage really well with the hands-on, active lessons.
An inquiry based activity that I would like to use in my classroom this coming year is the gravity and fallings things following the curious minds/esero framework. I really like that the key language is explicitly taught and that the children are given time to explore how forces make things move as an introduction to this topic (linking with play) The prompts are helpful and the whole framework is structured very well with starter questions and predicting. I feel that the pupils would enjoy conducting the investigation and interpreting the data and results. I feel like this framework lays out and progresses the inquiry based activity very well and I look forward to using it with my class.