While attending Lowell Celebrates Kerouac! 2008, Crystal and I made the necessary pilgrimage to Kerouac Park.
The park has a memorial to Jack consisting of pillars with engraved passages from his books about Lowell. Crystal liked the quote from Lonesome Traveler, "Am known as 'madman bum and angel' with 'naked endless head' of 'prose.'" This is my shadow (photo by Crystal) on that passage.
When you're in Lowell, definitely visit Kerouac Park.