The cost for you is marginal or non-existent, and you get an excellent referral system.
When you sell a membership, give the new member a bonus membership (one, three or even more months) he can gift to anyone he thinks might find it useful. You can test it as a surprise after they already joined, or as another incentive to join.
If your store caters to kids, make their shopping experience a delight, please their parents and have a chance to sell even more.
Offer a service to the parents where they order the kid’s present in advance. You gift wrap it for them and hide it somewhere in the store. You also write the kids name or otherwise mark their gift wrapped package with ink only visible in UV light. When they arrive, you give the child a magic wand (UV torch), and they go through the store searching for their hidden treasure.
Delighting your clients is always rewarding to you both.
Give “lucky bags” to clients when they finished buying and are about to leave (you can have a minimum transaction size.) These do not have to be expensive, but more of a novelty, and have more than 1 or two kinds. You can use these offline and online (by offering a selection of “bags” to choose from.)
To engage the visitor and on the way offer him opportunities to buy, and make sure they have fun and leave with good memories. The last impression before they leave is the most important, so this will help them remember you fondly.
When he enters the store, give him a small card with a good headline selling the first step, something like “You already have a surprise waiting for you. Just find our blue shelf.” There he’ll find the next step, maybe “The surprise is in our best selling book. You have two tries to find it. It’s on this shelf.” Inside the book there is a coupon and further teaser instructions (like: “If you want more on this subject, we have another surprise, go to our games section and look for the clue.”) Don’t only use coupons and discounts, do a raffle, give them a chance to pre-order something, just give them a small gift etc. also tell them they can ask for help from your staff.
You read a book. You think “my friend should read this book”.
You let us know what book you want to buy and to whom to send it to. You pay for the book. The friend who gets it doesn’t know that you sent it. He reads the book. If he likes it he might want to know who you are, so he pays for the book, and we tell him it was you who sent it. When he pays, you get credit for the full amount you paid for your next gifting pleasure. Or if you want to stay anonymous even after he pays he can send you in return a book you might like.