Relieved that his idea wasn't putting Luna out by making her shut up shop, Dominic smiled and offered her his arm. He apparated them immediately into his flat - right in front of the door to his spare room (study, workshop, experimental space - he could never quite decide what to call it). The door was shut so that Luna would get the full effect of the birthday message when she opened it. Let it never be said Dominic had no flair for presentation. "Go on," he said, gesturing to the door. "I didn't get you a present so this is it." He smiled again, because he did have some idea of shrinking down whatever the end result of the art-making was and sending it to her.
He didn't mention that he thought Dean was coming later. He knew through Seamus that sometimes Dean got caught up in his work and forgot to turn up to things. Dominic didn't want to promise Luna he'd be there if he ended up forgetting.