Give attention to hardware in the room. The switch plate cover for instance should be attractive. This gives a sense of quality. Especially in the foyer. If you can’t afford to replace your switch plate cover that’s okay, just make sure that it blends in. If the walls are painted, paint it to match. If they are papered, well you get the idea.
If you choose wallpaper with lots of pattern, you will have very little need for pictures or artwork. When choosing wallpaper, don’t assume that a small foyer should be papered with an equally small printed design. Some of the prettiest foyers I’ve been in, were small ones, where the paper chosen was large scale and very bold. The effect was wonderful! The entry is the perfect spot for tromp l’oeil. Possibly a mural of a Grecian urn or a large topiary.