I'd go for an open door with a visible staircase behind it. Ooh, or an elevator door! Open door with a Star Trek-style transporter room inside? Firehouse pole (ignoring the question of how one gets up said pole)?
I'd love a spiral staircase. To make the trap door useable from below, what about a springboard beneath it that we could use to bounce up and through it, like an insta-superbounce? Possibly with a spring noise...boi-oi-oi-oi-oinnng!