Two sites I discovered while researching St.John's Day include Fisheaters and Widow's Weeds. They are both Roman Catholic, and you have to weed through a lot of irrelevant/irreverent Popish doctrine, sometimes borderline paganism, but some of the lore and traditions are fun. Fisheaters especially does a good job (and an Augustine sermon is included in his St. John's post).
Also, this Latin teacher LOVES this (from fisheaters.com):
Another interesting thing about the Feast of St. John: the Breviary's hymn for this day, Ut queant laxis -- the hymn sung or recited during the blessing of the bonfire -- is the source of our names of musical notes -- Do, Re, Mi, Fa, So, La, Ti, Do. The hymn, attributed to Paulus Diaconus (Paul the Deacon, ca. A.D. 720-799), was noted by a monk to rise one note in the diatonic C-Scale with each verse. The syllables sung at each rise in pitch give us the names of our notes (the "Ut" was later changed to "Do" for easier pronunciation):
Ut queant laxis
Labii reatum, Sanc
And the melody is as follows:
(My Senior Thesis was on Paul the Deacon, to boot! Not promoting invocation of saints, here, just history :)
Two other customs I read about today include eating strawberries and making flower chains or wreaths! And we've done both--or at least made clover crowns and necklaces! I guess it helps that the clover's knee-high and strawberries are ripe :)
In addition to gathering St. John's wort, it's also customary to gather flowers to make wreaths to wear and to hang in your home or, especially, on the front door. In some places, such as Poland, some of these wreaths are floated down the river in honor of Christ's Baptism by St. John in the Jordan. Make a wreath of flowers that dry well, and hang in your home all year to be replaced next St. John's Day. Alternatively, flowers can be tied together in bunches with beautiful ribbons and hanged upside-down to decorate your home all year.