Since you and your girlfriend are about to start a long-distance relationship, I think you can give her some small gifts that can express your longing for each other and can be collected as commemorative gifts, such as color-changing couple cups or commemorative photo frames that can be printed with photos.
First, when your girlfriend is away at college, the chances of you meeting each other will inevitably decrease a lot. You can give her a wooden photo frame that can be customized with engravings, and then embed your sweet photos in it. If your girlfriend takes such a gift and puts it in the dormitory, she can often see you, which is a kind of sustenance.
Second, the same gift can also be this color-changing couple cup that can be printed with photos. Whenever you drink water, you can see each other's appearance. This can also be regarded as a kind of silent companionship, and giving a lover a cup means a lifetime.
Third, there are many gifts that represent longing for each other. In your case, you can also choose a couple ring to give to your girlfriend, the kind that can be engraved on it. This is a commitment to each other. At the same time, there are so many temptations in college, and this is also your declaration of sovereignty to your girlfriend, haha.