Hi! I don't know if this is silly...but I always like to take a green or flowering plant as a housewarming gift! I think plant liven up the place!
Anyway...you could do a cake like a flower pot and either do sugarpaste leaves (like a fern) or some flowers...would be SO cute! It doesn't have to be elaborate...just sweet! Or...you could do little pots with plants on top of a cake?? Good luck!