It may sound boring, but how about a couple of little shrubs that aren't going to look out of place even without flowers? A pair of hebes either side of the door, or some ivy trailing out of tall pots would look good. The advantage is that they don't need any care after a splash of water - and they are cheap! By spending the money saved on really showy pots that can move with you to the new place, you may even find yourself bitten by the gardening bug later in summer and already have a place to start!