Almost everyone has at least one person on their shopping list that seems impossible to buy for. That person may be incredibly picky; may go out and buy everything they want when they want it; or may be someone you do not know all that well.
For example, some couples who get married do not register for gifts because they already have their households established and do not feel they need any additional items. In that case, a gift card or gift certificate to one of their favorite restaurants, the theater, or for travel from an airline or travel agency would likely be appreciated. Or maybe the person you are buying the gift for is someone who lives far away and the gift you want to purchase is very bulky which would make shipping expensive. In that case a gift card or gift certificate for that item would work well.
A gift card can also be a great gift for someone you know extremely well. For example, my sister-in-law loves to get massages. But she has four children and a tight family budget so massages are a luxury she cannot usually afford. So for her birthday and for Christmas I give her gift certificates for massages or to a day spa. She loves receiving them!...