The use of marijuana by hippies is popularly known fact, it has been depicted in many Hollywood movies. It is also a highly effective way of making marijuana look innocent and benign to the eyes of teenagers. But let us also remember that cannabis is a very addicting drug that is believed to be a gateway drug to more potent and more dangerous drugs.
Even the hippies themselves moved on to the more potent drug called LSD after it was discovered and introduced in the mid 1960's. There is also psilocybin and mescaline which are also hallucinogens that the hippies used.
Amphetamines and opiates were also used in the hippie settings, though these harder drugs were disdained because they were harmful and addictive.