Hi Andrea. Some information is needed before anyone can make any useful suggestions: What is your soil type? What is the aspect? Is the site sheltered or exposed? What type of plants do you require (shrub, perennial, annual, ground-cover, evergreen, etc)? What height do you want them to be?

If you just want a temporary filler perhaps a few pelargoniums?

Presumably you want temporary fillers so your lupins have room for growth again next year - if its sunny, then pelargoniums would add some colour right now, as Steragram suggests, if its shady, not quite so much. Fuchsias maybe, its really a case of what you can find at the garden centre or nursery - they do still have some bedding left, but by now its in pots, is limited in range and will be more expensive.