I couldn't resist responding to this because I do the same thing. I have a small storage closet that has become the official "Gift Closet." When I see things on sale (after the start of school, after Christmas, etc.). I will buy them and put them in our Gift Closet. I try to choose items that are not trendy and have kind of a timeless appeal.
When my kids were younger I kept items like coloring books and crayons, swim goggles, games, etc. Also, I don't limit it to kid stuff. I will buy candle sets, fun coffee mugs, picture frames, etc. I buy Christmas trinkets after the holidays for teacher gifts for the following year. Plus, I always keep a supply of gift bags and tissue paper there as well.
Whenever the kids have a birthday party to go to, they go to the Gift Closet and "shop" for a present for their friend. I like it because they are able to choose something special specifically for their friend and don't have to just have to take the one item I happened to have on hand.
Granted, there have been times when the kids have grown past a certain age and a gift has not been given. In that case I either donate it (I feel ok because I know I got it at a great price and it is going to a worthy cause) or hold on to it in case I do have the opportunity to make it a gift.
The gift closet has saved us many last minute trips to the store for presents. It makes gift giving more affordable since you are buying items on sale, and yet still gives the opportunity for a personal present.