I find that the leafs() function helps me quickly create diverse test data to cover lots of BSON corner cases. I highly recommend using it.

In SBE, small strings are held in-line in slots, while large strings create a value on the heap, and store a pointer to that heap memory in the slot. Although leafs() is supposed to be BSON types that don't contain other values, it may be valuable to include a large string in this list.