When giving a gift give something that you would like to receive. Too many times companies will give away low quality gifts. When your customer receives a low quality gift from you they may perceive that all you have or do is low quality.
If your customer is willing to take time to drive past your competition and come to your store, surf past the thousands of web sites and spam ads to spend their hard earned dollars with you, then you need to take the time to put some thought and yes even some expense into a gift for them. Think of it as an investment toward future business.