Hey 
I mentioned already some simple covers Covers for empty holes? - #7 by AssetBurned a while ago. They do come in handy if you go on holiday and you have a small number of plants that you want to keep growing in the meantime. Of if you just can not decide on what to grow next.
I also saw some “adapters” to grow other types of pods, but I am not sure if that is really a good idea. Systems with other shapes of pods seems to require nutritions added to the water, which isn’t something I would suggest to the Click&Grow system.
But what might be another good idea is a custom lid for a mason jar that can hold a single pod. I know G&G is selling the tray holder for the 5 pods trays as well as the herb saver, but there isn’t really something for people using a 3 or 9 pod garden.