Playing kalimba is such a liberating activity! You seemingly have nothing but good choices in front of you. You can really cut loose and do whatever you want. Often, it works very well, because most kalimba tunings make success highly probable. And it's really fun to just let things flow.
You can also make great progress on the instrument by developing a new awareness - paying careful attention to something you love that you just came up with while playing, and then playing it again and again.
In my own experience, when a real gem (spontaneously or otherwise) comes to me on kalimba, I start out by NOT thinking much about it, just doing it, and repeating enough so that my body really knows it. Only then do I bring my mind on board to study what my thumbs have been doing.
After I begin to think, I quickly notate the song in tablature so I can capture this gem of a song. Then I can play it any time in the future, no regret for being too lazy to catch a beautiful inspiration.