Hi Maxime-
I have gone back and forth with the freebie issue too. I had been making handmade tags with handsewn fabric flowers adorning them (I sell them in my shop) but it was too much work and I am nearly out. So, after posting this same question in a forum and someone gave the idea of sachets (since I sew) and at the moment that is what I am giving out (also for sale in my shop). Again, too much work!
So once they run out, I have my best idea. I have ordered a stamp with my shop name and etsy address. I am going to just buy some manila tags, stamp them, use jute rope through to hang them and maybe place a ribbon bow. They will be able to be used as tags if its a gift and still advertise my shop and be cute!
I also put a business card, a nice card that I printed out at home with a handwritten thank you inside and a coupon code.