[This is what happens when they don't have babysitters, or at least supervision. TROUBLE. Poor Hiyori, stuck in a house with these two dweebs. At least they're fairly respectful. (???)]

Something cute is always best for Hiyori! Perhaps reminiscent of a mermaid, with shiny fabric and shells for decoration. That sounds great for Bishamon, though! She hardly needs anything to attract attention, but something to highlight her assets would really work well. Maybe frills in a cascade effect?