Start a writer's club - set it up at MeetUp.com...try for every other week. Give writing assignments or have them write a murder mystery with details about your store....
It will get people in and involved.
Start a Murder Mystery Club. Have one person read the entire book - everyone else is allowed to only read the first couple of chapters (the one who knows get's to decided where they have to stop). Then they meet to decide/argue who did it.
Advertise the clubs in craigslist or Meetup or local paper, see if local fast food would put up a flyer...
Have a treasure hunt. On a certain day hide clues in and around the books - winner get's....to pick out a free book with free coffee? Repeat a couple times over the course of the day? Prewritten hunts are available online for free...
Hold contests - where the contestants have to show up to win...Do a weekly question - (answer can be found in a book, in your store) again they have to be present to win and give away $50 (seems like a lot but money will attract quite a few people who impulsively buy)
Give coupons to neighboring stores - "As a thank you from Sally's HairDo Place have 10% off a coffee at the bookstore".
Have give aways if they purchase over $100 dollars in X month. Like a palm reading, a massage (those ten minute in a chair things), etc....
Hold classes...get guest speakers....amatuer poetry readings....
Have more fun.