Maybe not stereotypically hippy, but I recently discovered Sir Douglas Quintet, and that’s some really good music. Psychedelic Texas soul, great musicians, great songwriting, steeped in that cheesy 60s studio sound. Can you dig it?
My two cents: don’t fill up your playlist with tunes you can hear on any oldies station today. Try and find stuff the older folks may not have even heard since the 60s.
The* Austin Chronicle* devoted an issue to Doug after his death. Quite informative.
Doug started out as a country prodigy, on stage with Hank Williams in one of his last shows. Then he learned R&B in San Antonio, had a pop hit with “She’s About A Mover” & continued through Tejano, more R&B, stone country, swamp pop & a little bit of Cajun.
During his San Francisco years, he added a bit of psychedelia to his music. But just about any of his stuff would be a great choice…
Fantastic list, but it’s Steppenwolf. From the Hermann Hesse novel (Wolf of the Steppes)
Don’t forget In-A-Gadda-Da-Vida (the long version).
Yeah. 
The Youngbloods
Quicksilver Messenger Service
CSN&Y were not a 60s band. Remember, their first gig was at Woodstock in July 69, which was pretty much the tail end of the 60s.
For a “hippie” party, you might also consider
Creedence Clearwater Revival
Byrds
David Peel and the Lower East Side
Melanie
The Crazy World of Arthur Brown
Blue Cheer
Big Brother and the Holding Company
We middle class kids were listening to them in Junior High.
Our parents did not object. I don’t think they qualify as Hippie.
Good lists. But I’d add Sgt. Peppers. When hung out in the East Village about 1967-8 every head shop was playing “For the Benefit of Mr. Kite” all the time.
Pinball Wizard—The Who
I’d consider any song that was great to listen to while stoned was a “Hippie” song. And trust me, under that condition, “Born on the Bayou” qualified. 
Donovan - mellow yellow.
Please throw in some Charlie Manson, too.
“Teach Your Children” Crosby Stills and Nash - one pictures a video - handheld camera - of a bearded hippie, his “old lady” with a headband, breastfeeding the newest infant, and two or three toddlers running around stark naked in a barnyard.
“Marrakesh Express” Crosby Stills and Nash - all those free-wheeling upper middle class “hippies” off backpacking on a summer adventure, a graduation present (on the parents’
dime, of course). How nice to be off on your travels to get high, not a care in the world, no worries money or otherwise!
Procol Harum’s A Salty Dog
Traffic – Dear Mr. Fantasy. I really want to include John Barleycorn Must Die, though that didn’t come out until 1970
Grand Funk Railroad – “I’m Your Captain/Closer to Home”
There is also Stephen Stills "Love the One You're With". Rich musician is on tour and separated from his girlfriend....solution-have sex with available woman.
On a more serious note for the OP: Country Joe and the Fish “Electric Music for Mind and Body”.
And their Magic Bus.
How can you not mention “A Whiter Shade of Pale”?
And although everyone tries to wipe it from their minds, back then, Iron Butterfly’s “Inna Gadda Da Vida” certainly qualified.
Clyde often played it on Beaker Street, so I think it qualifies.
Gotta be Pig-Pen era.
Quicksilver Messenger Service, Hot Tuna, crap like that.
Pig-Pen certainly, but I’d still include some Brent songs, for sure.
Oh, and don’t forget Ultimate Spinach.
Even if you did, though, I’d totally hang all night to Dead shows, easy!