It’s tempting, as “Stranger Things” lumbers towards a final season so long delayed the kids will probably have their AARP cards, to start finding new releases to fill the gaps. We’re talking purestrain nostalgia for the light-hearted kids’ films of the ’80s, whether they be Stephen King adaptations or Spielberg productions like “The Goonies.” “Monster Summer,” the micro-budgeted but big-hearted adventure from child-actor-turned-director David Henrie (he recently played a younger Reagan in the biopic of the same name), does the trick well enough, even as it doesn’t do much beyond play the witchy hits.