Perhaps the English language is so limited in its lexicon that there just wasn't a word that sufficiently described what the committee was aiming for. Though this may be the case, I would still like to make a suggestion. Instead of academic rigor we should strive for academic (wait for it; wait for it).
One possible definition is unimpeded scope or opportunity for action. Granted that is pushing the connotation of play, but hey if Merriam-Webster says it, then it must be true. I think however that we could all agree that when we hear the word play we definitely think of the "spontaneous activity of children." Why not work with this spontaneity and make children spontaneous learners.
Good teachers do this anyway, but because of NCLB and our ridiculous obsession corporate and global economies we are forced to talk about education with words that bring forth images of stiffening corpses.
So abolish rigor. Abolish work. Abolish the animated corpse that has a stranglehold on education. Abolish homework. Establish homeplay. Abolish worksheets. Establish playsheets. Establish playbooks. Establish group play.
When a child says that their favorite part of school is recess they should be referring to American Lit., Algebra II, Chemistry, and World History.