YMCA? Bring a heavy medicine ball from the weight room to the large gymnasium area where they tend to keep gymnastic mats. Slam it on there so it A) won't be super loud and B) won't eventually destroy the medicine ball.

I think Toes-to-bar can be a great alternative, he engage the whole "abs packet" in one mouvement and he can improve your shoulders and core strenght useful for abs roller and planks.