Granted the van and Shaggy's hippy style could simply be labled a product of the times (late 60's/early 70's) but another running theme is that Shaggy & Scooby seemed to have the "munchies". Is it harmless coincidence or the creators' inside joke?
I think HB just wanted to have their cake and eat it too, here. Use elements with popular current feel while avoiding any negative elements associated with it. Hip but safe. I seriously doubt anyone in any position of authority there intentionally put drug references in the show or allowed them to slip through if they could catch them, although it might have been possible that writers might have put the occasional obscure ones in scripts that they didn't catch.
This included smoking and drinking; Joe Barbera once was said to be totally anti-nicotine due to his having asthma, and drugs got hardcore jazz musicians in as much trouble exactly as they did the people doing rock..Bugs wasn't considered entirely anti-establishment...Chuck Jones's cartoons with him tended to suck as Bugs was a goody goody in Chuck;s hands in the 40s and against in the 5o0s. Now Bob Clampett and the king, Text Avery, their Bugs was groovy!In his autobiography, Iwao Takamoto stated uncategorically that they are not hidden or veiled drug references in Scooby-Doo. He noted that in the late 60's, you simply did not do anything that could be construed as a drug reference if you wanted to stay on the air, and added that "drugs of any kind were an anathema to Joe Ruby; he hated them" .