a mirror adjacent to the door makes the entrance seem wider. don't put anything behind the door, because it needs to be able to open all the way.
use something musical to attract energy in. i have two bronze bells on hooks on the the inside of my front door, and the sound always makes me happy. (plus whenever i order take-out, i tuck the money into one of the bells, so it's right there no matter who answers the door!) you could use windchimes or a strand of tiny bells over the doorknob. it doesn't sound like you have room for a fountain.
you could also make the entrance to your home more attractive from the outside, like by placing identical planters on either side of your door, or sconces, or . . . painting the door? the standard advice is red. my door is still white. i haven't decided on a color. also just keeping it very tidy and removing any obstacles will help. sweep the steps/sidewalk, etc.