How do I expand my mind a little bit, man?

You need to be wearing the Filthy Hippy Disguise, which is a set with the filthy knitted dread sack and the filthy corduroys. Both of these items are dropped by hippies in the Hippy Camp.

Another option is the War Hippy Fatigues (reinforced beaded headband, bullet-proof corduroys, and round purple sunglasses), but you will typically have the equipment for the Filthy Hippy Disguise by the time you have the capability of getting the War Hippy Fatigues.

Note that if you clear the infestation at the Hippy Store (a side-quest during the Island War main quest), you will be able to access the Hippy Store without having to wear a specific set (and you'll gain ~4000 meat every day by visiting).