So in addition to the K'nex blind bag figures, there's also some 3 packs which feature a Mario variant, a second figure and a blind figure. Other Sets include fire Mario with hammer brother, Gold Mario with Fire Luigi. The internet says the former comes with a bob-omb and the latter comes with a gold bullet bill. The fact that this pack comes with a boomerang plant tells me the mystery figures are props from the building sets.
Seriously, the boomerang plant doesn't even stand up on its own. It's missing some sort of base or stand or something that seems to go with one of the Mario K'nex building sets. But $10 for 2 figures and a large prop, when singles usually go for $4, and I can't say I'm disappointed.
I really Want to get a hammer brother now but I'm pretty sure that it will eventually come out as a blind bag figure in a future wave.