I assume (because you say "1970 or so") you've heard Open Road. It's not essential but it's worth hearing; Cosmic Wheels is kind of in the same boat except that it's not nearly as good. I haven't heard it in forever but the songs that immediately come to mind as good are the title track and "I Like You". The latter is particularly good though awfully produced (I was listening to a lot of Donovan when I got my first girlfriend and associate the song strongly with feelings and memories of that time; the song might not actually be that great, boosted merely by nostalgia). There's also "The Intergalactic Laxative" so there's that. There's an attempt to get more glam-rocking and it's basically the equivalent of Sparks' Big Beat in places (better IMO).
I think these three albums are the first of the '70s. After them there's a sharp dropoff and he released really nothing essential until Sutras in 1996. Of them maybe the most notable is his self-titled, intended as a comeback, but it didn't take off (didn't deserve to, TBH).