I found M. fully clothed in the shower (water off, thankfully) having a lot of fun drumming on the glass with her hands. Although my initial reaction was to get her out I decided to seize the moment for a bit of II instead and it was great!
We often take turns drumming on the wall with our hands but a limitation of this is that we're both facing the wall so eye contact is problematic unless you actually turn your head sideways. The shower overcame this problem because I could align myself on the other side of the glass, meaning that not only was eye contact easy but also we could align our hands together too. When the interaction was established I tried developing it by alternating loud drumming with gentle finger tapping (she reciprocated) and more rhythmic drumming to accompany a sung commentary about her actions. She listened to that with interest and smiled but didn't feel like joining in.